Introduction

Top Abstract Introduction Materials & Methods Results Discussion References

Taurine is a sulfonic amino acid ubiquitously and abundantly distributed in tissues of numerous animal species. The high concentrations parallel its involvement in many physiological processes such as osmoregulation, calcium mobilization and antioxidant action (Huxtable, 1992). Taurine is essential for normal development and proper function of the excitable tissues of mammals (Huxtable, 1992; Pion et al., 1987; Sturman, 1993). As far as skeletal muscle is concerned, taurine plays a fundamental role in the electrical stabilization of cell membrane (Conte Camerino et al., 1987). In fact in vitro application of taurine on skeletal muscle fibers reduces fiber excitability by specifically increasing GCl (Conte Camerino et al., 1987), the parameter that mostly contributes to the electrical stability of sarcolemma (Lehmann-Horn and Rüdel, 1995). We have proposed that this effect is mediated by the interaction with a low affinity site controlling chloride channels, because taurine analogs are less effective on GCl (Pierno et al., 1994).

The physiological role of taurine in skeletal muscle has been more clearly demonstrated by the alterations of the electrical and contractile properties consequent to an experimentally produced taurine deficiency. Chronic administration of GES, a competitive inhibitor of taurine transporter (Huxtable, 1992), causes a fall in muscular taurine content and produces a marked decrease of GCl along with an increase of excitability of rat skeletal muscle fibers (De Luca et al., 1996a). Furthermore the excitation-contraction (e-c) coupling process of taurine-depleted muscles is significantly changed, the contraction occurring at more negative potentials with respect to normal controls (De Luca et al., 1996a). We have demonstrated that the alteration of mechanical threshold in taurine-depleted muscle is not caused by the decrease of GCl, leading to hypothesize that taurine controls this function through other mechanisms (De Luca et al., 1996a). For instance Huxtable and Bressler (1973) found that taurine enhances the rate of Ca⁺⁺ and the total Ca⁺⁺ sequestering capacity of sarcoplasmic reticulum isolated from rat skeletal muscle. This observation allowed us to propose a possible impairment of this process in taurine-depleted muscle that could result in an increase of cytosolic Ca⁺⁺ responsible for the alteration observed in the l-c coupling mechanism (De Luca et al., 1996a).

Taurine content declines slowly during late adulthood and decreases further during aging. Reduced taurine levels have been found in plasma, and some other tissues (atria, kidney and caudal artery) of 30-mo-old male Fischer 344 rats with respect to those of 8-mo-old rats of the same strain (Dawson and Wallace, 1992). Several abnormalities in the morphology and function of skeletal muscle have been reported during aging (Carmeli and Reznick, 1994). Among these we have found alterations in the electrical and contractile properties closely resembling those occurring in taurine-depleted muscle. We observed a specific reduction of GCl and a shift of the mechanical threshold toward more negative potentials in striated fibers of aged rats (De Luca and Conte Camerino, 1992; De Luca et al., 1992, 1994). All these findings led to us to hypothesize that a reduction of muscle taurine content could occur during aging and this can play a role in the age-related changes of skeletal muscle function. We tested this hypothesis by evaluating the changes of taurine content in blood and skeletal muscle of aged rats and the effects of an in vivo treatment with taurine on the ionic conductances, excitability parameters and mechanical threshold of EDL muscle of aged rats. To better evaluate the potential benefit of the pharmacological treatment with this amino acid in the aged subject and to shed light on its mechanism of action we also performed in vitro pharmacological characterization of GCl of taurine-treated animals with the R-(+) enantiomers of CPP. In fact the age-related decrease of GCl is accompanied by a change in its sensitivity to this specific channel ligand so that R-(+) CPP produces different effects in aged vs. adult animals (De Luca et al., 1992, 1996b). Also, by testing the sensitivity to phorbol esters in muscles of taurine-treated aged rats we evaluated the possible effect of taurine on the PKC-mediated phosphorylation of the chloride channels. The 4--phorbol 12,13-dibutyrate, able to activate a Ca⁺⁺ and phospholipid-dependent PKC, is almost 20-fold more potent in reducing GCl in aged than in adult muscles suggesting an alteration during aging of the biochemical pathways modulating channel function (De Luca et al., 1994). Recent studies have shown that taurine, by reducing cytosolic Ca⁺⁺ levels and by inhibiting phosphoinositide turnover may inhibit PKC-catalyzed phosphorylation processes in rat brain (Li and Lombardini, 1991). For comparison the effects of taurine treatment were also evaluated on the adult (6-mo-old) rats.

	Materials and Methods

Top Abstract Introduction Materials & Methods Results Discussion References

Animals and Taurine Administration

Ten adult (6-mo-old) and 18 older (22-mo-old at the beginning of the experimental period) male Wistar rats of 400 to 500 g (Morini Laboratories, S. Polo D'Enza, Italy) were used for all experiments. The animals were maintained one per cage, with free access to food (Charles River, Calco, Italy, 4RF21), at a constant room temperature (20-22°C) and exposed to a light cycle of 12 hr/day (8.00 A.M.-8.00 P.M.) throughout the course of the experiments. Rats were subdivided in four groups: taurine-treated adult (n = 5) and taurine-treated aged (n = 9) rats receiving 1 g/kg taurine/day (Teofarma, Pavia, Italy); untreated adult (n = 5) and untreated aged (n = 9) rats receiving tap water were used as control. Taurine, dissolved in drinking water (2%) with sucrose (2%) was administered to the rats until they drunk the daily dose of 1 g/kg, contained in a volume of 20 to 25 ml. For the rest of the day they had free access to taurine-free drinking water. This treatment lasted 2 to 3 mo. Control rats also received sucrose (2%).

Tissue Preparation and HPLC Analysis

Trunk blood was collected in centrifuge tubes rinsed with 10 µl of ethylendiaminetetraacetic acid (150 mM). Part of blood was stored at 80°C until assay for taurine determination (Huxtable, 1992), and the other fraction was used for measuring glucose by hexokinase enzymatic method (Bondar and Mead, 1974). Tibialis anterior muscles were removed, washed in physiological solution, dried, weighed and homogenized with 10 ml of HClO₄ (0.4 N) per g tissue. The homogenized muscles were buffered with 80 µl K₂CO₃ (5.5 g/10 ml) for each ml of HClO₄ used. The homogenates were centrifuged at 600 × g for 10 min at 4°C. The supernatants were stored at 80°C until assay. Derivatization with o-phthalaldehyde was performed as previously described and samples were processed for HPLC taurine determination (Lleu and Huxtable, 1992).

Electrophysiological Experiments

Measurements of cable parameters, ionic conductances and excitability characteristics. Electrophysiological measurements were made in vitro at the end of the treatment period. The EDL muscles of both hindlimb were dissected under urethane anesthesia (1.2 g/kg, i.p.). Soon after the removal of the muscles the rats, still anesthetized, were killed by further i.p. injection of a urethane overdose. The muscles were placed in 25 ml muscle bath, maintained at 30°C and perfused with normal or chloride-free physiological solutions (Bryant and Conte Camerino, 1991) as detailed below. The membrane properties were obtained with the two intracellular microelectrode current clamp method in which a hyperpolarizing square-wave current pulse is passed through one electrode and the membrane voltage response is monitored at two distances from the current electrode (Bryant and Conte Camerino, 1991). The current pulse generation, the acquisition of the voltage records and the calculation of the fiber constants were done in real time under computer control as described in detail elsewhere (Bryant and Conte Camerino, 1991). From the experimentally determined values of input resistance, space constant and time constant and from an assumed myoplasmic resistivity (Ri) of 125  × cm for both adult and aged fibers in agreement with previous studies (Boyd and Martin, 1959; De Luca et al., 1992), calculated fiber diameter (dcalc), membrane resistance (Rm) and membrane capacitance (Cm) were then calculated (Bryant and Conte Camerino, 1991). The reciprocal of Rm from each fiber in normal physiological solution was assumed to be total membrane conductance (Gm), and the same parameter measured in chloride-free solution was considered to be potassium conductance (GK). The mean chloride conductance (GCl) was calculated as the mean Gm minus the mean GK. The excitability characteristics of the sampled fibers were determined by recording the intracellular membrane potential response to square-wave depolarizing constant current pulses. In each fiber the membrane potential was set by a steady holding current to 80 mV, before passing the depolarizing pulse (Pierno et al., 1994).

Measurements of mechanical threshold. The mechanical threshold of the fibers was determined using a two microelectrode "point" voltage clamp method as previously described (Dulhunty, 1988; Heiny et al., 1990; De Luca and Conte Camerino, 1992). In brief, a voltage-sensing electrode (3 M KCl) and a current-passing electrode (2 M potassium citrate) were inserted within 50 µm of each other into the central region of a randomly selected superficial fiber that was continuously viewed using a stereomicroscope (100 × magnification). The holding potential was set at 90 mV and depolarizing command pulses of variable duration were given at a rate of about 0.3 Hz. Tetrodotoxin (3 µM) was continuously present during recordings to prevent action potential generation (Dulhunty, 1988; Heiny et al., 1990; De Luca and Conte Camerino, 1992). As a standard protocol the command-pulse duration was usually set sequentially to each of the following values: 500, 50, 5, 200, 20, 100 and 10 msec. At each duration, the command voltage was increased using an analogue control until contraction was just visible, and then backed down until the contraction just disappeared. A digital sample-and-hold millivoltmeter stored the value of the threshold membrane potential at this point. We estimated the uncertainty of any single measurement for a given fiber to be 1 to 2 mV. Particular care was taken to perform the measurements in any experimental condition in an identical fashion, with about the same length of time involved in each determination so as to exclude any effect on the mechanical threshold of intracellular citrate ions from the electrodes (Dulhunty, 1988). The threshold membrane potential V (mV) for each fiber was averaged at each pulse duration t and then the mean values plotted against duration gave us a "strength-duration" relationship. A fit estimate of the rheobase voltage (R) and of the time constant to reach the rheobase was obtained by a nonlinear least square algorithm using the following equation: where H is the holding potential (mV), R is the rheobase (mV) and  is the time constant (msec) (Miledi et al., 1983; De Luca and Conte Camerino, 1992). In the fitting algorithm, each point was weighed by the reciprocal of the variance of that mean V and the best fit estimates of the parameters R and  were made. The mechanical threshold values are expressed as the fitted rheobase (R) parameter ± S.E. which was determined from the variance-covariance matrix in the nonlinear least square fitting algorithm.

Solutions and Drugs

The normal physiological solution had the following composition (in mM): NaCl 148; KCl 4.5; CaCl₂ 2.0; MgCl₂ 1.0; NaHCO₃ 12.0; NaH₂PO₄ 0.44 and glucose 5.55. The chloride-free solution was prepared by equimolar replacement of methylsulfate salts for NaCl and KCl and nitrate salts for CaCl₂ and MgCl₂. Both solutions were continuously gassed with 95% O₂ and 5% CO₂ (Bryant and Conte Camerino, 1991). To suppress spontaneous contraction of muscle preparations 1 µM tetrodotoxin was added to the chloride-free solutions. The pH of all the solutions used was carefully maintained between 7.2 to 7.3 during each experiment. To test the effects of R-(+) 2-p-(chlorophenoxy) propionic acid (R-(+)-CPP), stock solutions were prepared in 1% sodium bicarbonate solution. The final concentration was obtained by further dilution in normal physiological solution (De Luca et al., 1992). 4--phorbol 12,13-dibutyrate (4--PDB; Sigma Chemical Co., St. Louis, MO), was dissolved in DMSO to produce concentrated stock solutions, to be added in microliter amounts to the bath solutions, as needed. A 0.5% DMSO solution, much stronger than the maximum DMSO concentration used (0.04%), was without effect on the parameters studied (De Luca et al., 1994). The 4--PDB was tested at different concentrations (from 3 to 50 nM), no more than three doses being tested in each preparation. The time of incubation of PDB was varied from 90 min (3 nM) to 30 min (50 nM) so as to reach a steady-state of drug effect (De Luca et al., 1994).

Statistical Analysis

The concentrations of taurine in blood and muscle are expressed as mean ± S.E.M. from N number of animals. The electrophysiological data are expressed as mean ± S.E.M. from n fibers of N EDL muscle preparations. The estimates for S.E.M. and N of GCl were obtained from the variance and from the number of fibers sampled for Gm and GK as described by Green and Margerison (1978). Significance between groups of means was evaluated by Student's t test. The IC₅₀ values for phorbol esters were estimated by fitting the logistic function to data points, as described in detail elsewhere (De Luca et al., 1994). The statistical significance between the fitted values of rheobase was estimated by a Student's t distribution, using a number of degrees of freedom equal to the total number of threshold values determining the curves minus the number of means minus two for the free parameters (De Luca et al., 1996a). Statistical differences between untreated aged and taurine-treated aged rats groups were also evaluated for significance using analysis of variance.

	Results

Top Abstract Introduction Materials & Methods Results Discussion References

General observations. Taurine treatment did not modify food consumption or body weight gain either in aged or adult animals. The 18 older rats used for the experiments were in good health with no impairment of hind limb movements or locomotor activity and no pathological sign was observed in any group of rats throughout the period of study. No mortality was observed in both taurine-treated and -untreated aged rats. However, blood glucose was found to be increased by 155 and 83% in untreated aged and taurine-treated aged rats, respectively, with respect to the adult group. The insulin-like action of taurine (Huxtable, 1992) may account for the reduced increase in blood glucose found in taurine-treated aged rats.

Plasma and skeletal muscle taurine content in adult and aged rats before and after chronic administration of taurine. As shown in table 1 tissue taurine content (determined by HPLC) was significantly lowered by 25% in the tibialis anterior muscle of untreated aged rats with respect to the untreated adult rats. In aged rats taurine administration significantly raised the muscular levels of the amino acid to the values found in the adult rat muscles. In contrast taurine treatment did not modify the taurine content in the muscle of adult rats. Similar taurine blood levels were found in both untreated adult and aged rats. The administration of taurine significantly increased the blood levels of taurine in the aged rats.

Effects of taurine chronic administration on the membrane ionic conductances and excitability parameters of muscle fibers from adult and aged rats. In agreement with previous findings (De Luca et al., 1992; 1994) Rm of EDL muscle fibers was significantly higher in aged with respect to adult rats (395 ± 16  × cm², n = 106 and 335 ± 8.5  × cm², n = 74, respectively, P < .005). The increase in Rm reflected a significant decrease of Gm that was almost completely attributable to a 16% decrease of GCl found in the aged rats (table 2). The GK was slightly higher in the aged with respect to the adult rats, as frequently occurs during aging (table 2) (De Luca et al., 1994). Daily chronic treatment with 1 g/kg taurine produced a significant decrease of Rm in the 24-mo-old rats with a mean value of 329 ± 7.6  × cm² (n = 112) and a consequent restoration of GCl vs. the adult value (table 2). As shown in table 2, this effect occurred with a high incidence, indeed a significantly higher value of GCl with respect to the value found in the untreated aged rats was observed in six of seven aged treated rats studied. The seventh rat was unaffected by the treatment. The mean GK was not significantly modified by taurine treatment (table 2) and in one taurine-treated animal a significantly higher value of GK was observed. In the adult rats the taurine treatment slightly, but significantly, increased GCl by 10% with respect to the adult untreated rats, without modification of GK (table 2).

Effects of R-(+) enantiomer of 2-(p-chlorophenoxy) propionic acid on membrane chloride conductance of muscle fibers from untreated and taurine treated aged rats. It has been demonstrated (De Luca et al., 1992) that GCl and consequently chloride channel function can be stereospecifically modulated by drugs such as the R-(+) enantiomer of CPP. As shown in figure 1 in adult rats the R-(+) CPP produced a typical biphasic effect, increasing GCl at low concentrations (3 µM) and decreasing it at higher concentrations (40-100 µM). In contrast, on four muscles from four aged rats, R-(+) CPP did not produce the typical biphasic response, but increased GCl at all doses tested (fig. 1). Interestingly the in vitro application of R-(+)-CPP on four muscles of four aged rats chronically treated with taurine restored the typical biphasic response observed in adult rats (fig. 1).

View larger version (38K): [in this window] [in a new window]   Fig. 1.   Effects of in vitro application of the R-(+) enantiomer of 2-(p-chlorophenoxy) propionic acid (R-(+) CPP) on resting chloride conductance (GCl) of extensor digitorum longus muscle fibers of adult, aged and taurine-treated aged rats. Each bar represents the mean value of GCl obtained from 16-61 fibers of 2-4 muscle preparations. * Significantly different with respect to the related control values in the absence of R-(+) CPP (open bars; without drug) (P < .05 or less).

Effects of phorbol esters on membrane chloride conductance of muscle fibers of untreated and taurine treated aged rats. The in vitro application of 4--PDB, a well known PKC activator, tested in the range from 3 to 50 nM, produced a concentration-dependent block of GCl which was much greater in EDL muscle fibers from four aged rats than in those from adults (fig. 2). Indeed the concentrations required for half-maximal block of GCl (IC₅₀) were 25.6 ± 1.7 and 9.06 ± 0.44 nM in adult and aged EDL muscle, respectively. Moreover at a concentration of 50 nM, 4--PDB produced a complete block of GCl (99%) in aged muscle fibers, whereas the same concentration produced a 76% block of GCl in the adult (fig. 2). The block of GCl produced by 4--PDB in four taurine-treated aged rats was similar to that observed in the adult, particularly at the lower concentrations (3 and 10 nM), with an IC₅₀ of 18.4 ± 0.6 nM (fig. 2). The in vitro application of 4--PDB on EDL muscle fibers from taurine-treated adult rats produced a concentration-dependent block of GCl similar to that found in the adult untreated rats, except for the two lower doses, which were less effective in blocking GCl (fig. 2). Thus, the half-maximal concentration of 4--PDB in taurine treated adult rats was 31 ± 1.1 nM.

View larger version (13K): [in this window] [in a new window]   Fig. 2.   Effects of in vitro application of 4--phorbol 12,13 dibutyrate (4--PDB) on resting chloride conductance (GCl) of extensor digitorum longus muscle fibers from untreated adult and aged rats (filled symbols) and taurine-treated adult (Adult + Taurine) and aged (Aged + Taurine) rats (open symbols). The mean values of GCl obtained at each concentration of 4--PDB (from 16-35 fibers of 2-4 muscles) have been normalized vs. the group-related mean values of GCl recorded in the absence of phorbol ester. Thus each point is the normalized percent change ± S.E.M. of GCl in the presence of 4--PDB.

Effects of taurine chronic administration on the mechanical threshold of muscle fibers of aged rats. The threshold potential for contraction of extensor digitorum longus muscle fibers from both control and taurine-treated rats showed the typical dependence on command pulse duration; i.e., it was the more negative the longer the duration of the pulse. Under the experimental conditions used (t = 30°C and rate of about 0.3 Hz), a constant rheobase value was almost fully reached at the longest pulses used, a behavior commonly seen with mammalian muscle fibers (De Luca and Conte Camerino, 1992; De Luca et al., 1996a). In line with previous results (De Luca and Conte Camerino, 1992), in our experiments the aged fibers needed significantly less depolarization to contract with respect to those of adult at each pulse duration, and the resulting strength-duration curve obtained from five aged rats was clearly shifted toward more negative potentials with respect to that of the four adult animals tested (fig. 3). The voltage at rheobase (R) estimated from the fit of the experimental points was significantly different with respect to that of the adults, although the time constant () to reach the rheobase was slightly longer, although not significantly, in the aged rats compared to the adults (table 3).

View larger version (16K): [in this window] [in a new window]   Fig. 3.   Strength-duration curves for the threshold potentials for mechanical activation of extensor digitorum longus muscle fibers from untreated adult (number of animals = 4) and aged (N = 5) rats (adult and aged; continuous lines) and taurine-treated adult (adult + taurine; N = 4) and aged (aged + taurine; N = 5) rats (dashed lines). Each point is the mean value ± S.E.M. of the threshold potential (in mV) recorded at each pulse duration from 18 to 48 fibers. The curves fitting the experimental points have been obtained using the equation described in "Methods." The fitting procedure allowed also to obtain the following calculated values ± S.E. of the rheobase voltage (R) and of the time constant () in each experimental condition showed in table 3.

	Discussion

Top Abstract Introduction Materials & Methods Results Discussion References

Previous results obtained with aged Fisher 344 rats suggested that during aging the plasma taurine level may fluctuate in relation to dietary intake and renal function, whereas tissues such as brain and heart can retain more stable taurine levels provided that the high-affinity taurine transporter works properly (Dawson and Wallace, 1992). Our experiments performed on aged Wistar rats have shown no decrease in taurine content in the blood but a significant reduction of its level in skeletal muscle with respect to adult rats. Chronic taurine administration markedly raised taurine levels in blood and produced a significant increase of the amino acid muscle content toward the adult value, showing that the tissue fall can be pharmacologically counteracted. These findings suggest that the age-related decline in muscle taurine content may be due to alterations of the transport system responsible for the uptake of taurine inside the fibers (Huxtable, 1992; Jhiang et al., 1993). One possible mechanism for the reduced taurine influx during aging can be related to the biochemical modulation of its transporter. In fact recent studies in Ehrlich cells have proposed that the transport -system accounting for taurine accumulation, is inhibited after phosphorylation on specific sites by phorbol esters-activated PKC (Mollerup and Lambert, 1996). It is notable that an overactivity of PKC can occur in skeletal muscle fibers during aging (De Luca et al., 1994). A volume-activated taurine efflux through selective channels, already described in many tissues (Kirk and Kirk, 1993; Ballatori et al., 1995; Moorman et al., 1995), may be also hypotesized.

Whatever the mechanism underlying the cellular decrease of taurine during aging, our results show that it plays a role in the alteration of the electrical and contractile properties observed in this situation. Interestingly from a therapeutic point of view, the chronic treatment with the amino acid, raised the muscle taurine content vs. the adult value, although functional parameters were ameliorated. The taurine treatment improved the e-c coupling mechanism, i.e., the rheobase voltage for contraction, abnormally negative in striated fibers of aged subjects, was shifted toward the adult value. In parallel, the lower GCl found in old rats was increased toward the adult value and the excitability parameters related to GCl were similarly ameliorated. The ability of intracellular taurine level to control both the mechanical threshold and GCl of muscle fibers had been already observed by inducing a pharmacological taurine deficiency in rats (De Luca et al., 1996a). The lowering of mechanical threshold could be in part related to the decrease in GCl either affecting the membrane resistance or enhancing the calcium entry during the prolonged action potential duration (Bianchi, 1992). The contribution of the action potential duration cannot be evaluated in our recordings of mechanical threshold for the need to block action potential rise with tetrodotoxin, although we have previously ruled out the contribution of changes of membrane resistance, due to low GCl, in the alteration of mechanical threshold (De Luca et al., 1996a). All these observations open two main possibilities for the mechanism of action of taurine on muscle function: the intracellular taurine content can affect independently different cellular functions or rather act on a unique step able to modulate various effectors. Our findings and the data available in the literature favor the latter hypothesis. It has been long claimed that taurine exerts a direct modulatory effect on the e-c coupling in the heart by acting on Ca⁺⁺ availability for contraction (for review see Huxtable, 1992). In skeletal muscle taurine has been found to stimulate Ca⁺⁺ uptake and storage capacity of sarcoplasmic reticulum (Huxtable and Bressler, 1973). A direct ability of taurine to increase Ca⁺⁺ binding to membrane phospholipids has also been observed (Huxtable, 1992). Furthermore in muscle of aged rats we found a reduction of taurine content while Larsson and Salviati (1989) described an impairment of the Ca⁺⁺ sequestration capacity of sarcoplasmic reticulum, resulting in higher level of cytosolic Ca⁺⁺. The relationship between cytosolic Ca⁺⁺ and mechanical threshold is supported by the finding that in vitro application of the calcium ionophore A23187 to striated fibers shifts contraction toward more negative potentials (Morgan and Bryant, 1977). These observations corroborate that a taurine deficiency, such as that occurring naturally during aging, leads to an increase in cytosolic Ca⁺⁺ able to affect e-c coupling mechanism, and that this situation can be counteracted by the amino acid administration through a restoration of intracellular taurine content. The proposed increase of Ca⁺⁺ availability, occurring in taurine-depleted muscles of aged rats, can in turn account in a positive feed-back loop for the decrease in GCl observed in this situation. In fact GCl is reduced by application of the calcium ionophore A23187 as well as by the activation of a Ca⁺⁺ dependent PKC (De Luca et al., 1994). If this hypothesis is correct one would expect an ability of taurine to act on the biochemical modulatory pathway of the chloride channel (De Luca et al., 1994). Accordingly, we found that the taurine administration reduced the potency of 4--PDB in decreasing GCl in aged rats, so that the block of GCl resulting from the phorbol ester-induced-PKC activation was similar to that observed in normal adults. Moreover Li and Lombardini (1991) found that taurine inhibits PKC catalyzed phosphorylation processes in rat brain by reducing the cytosolic Ca⁺⁺ levels and by inhibiting phosphoinositide turnover. The taurine treatment also restored the sensitivity of the chloride channels to a specific channel ligand. As generally observed in adult rats, the R-(+) CPP produced in taurine-treated aged muscle an increase of GCl at low concentrations and a decrease of it at the higher ones, in contrast with the lack of this biphasic response observed in aged untreated muscle (De Luca et al., 1992). We have recently observed a suppression of the biphasic response by the R-(+) CPP in adult animals after muscle pretreatment with phorbol esters, i.e., after induction of channel phosphorylation, suggesting that the phosphorylated channel may have a different pharmacological sensitivity (De Luca et al., 1996b). These findings again corroborate the ability of the taurine treatment to restore chloride channel function and pharmacology by acting on Ca⁺⁺ and PKC-mediated biochemical pathway.

A direct action of taurine on chloride channels can also contribute to the effects observed after chronic treatment. In fact the administration of taurine to adult rats increased the amino acid level in blood but not in skeletal muscle and no remarkable effects were observed on the electrical and contractile properties. However the taurine-treated adult rats showed a slight increase of GCl along with the modification of the related excitability parameters, similarly to what observed when taurine is applied in vitro on skeletal muscle. Furthermore, lower concentrations of 4--PDB were less effective in reducing GCl of taurine-loaded adult rats with respect to untreated adults and this is also observed when 4--PDB is applied in vitro on adult EDL muscle previously incubated with taurine (18-30 mM) (unpublished observations). We already described that in vitro application of taurine in the mM range produces a specific increase of skeletal muscle GCl, by acting on a low-affinity site (Conte Camerino et al., 1987; Pierno et al., 1994). Thus a direct pharmacological action of taurine on chloride channel of aged rat muscle, synergic with the effects produced by the restoration of the muscle taurine content, cannot be excluded. At the moment the verification of this hypothesis is made difficult by the fact that the chloride channels accounting for the large GCl in skeletal muscle cannot be studied by patch clamp methodology in native muscle fibers (Pusch et al., 1994).

Our results add new evidences about the importance of maintaining appropriate level of intracellular taurine for skeletal muscle function. In particular the aminoacid supplementation may ameliorate muscle performance in aged subject. Taking into account that this physiological compound is almost free of side effects, the results of our in vivo studies corroborate its therapeutical potential in pathophysiological situations such as aging.