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NEUROPHARMACOLOGY

Facilitation of the Purinergic Contractile Response of the Guinea Pig Vas Deferens by Sodium Orthovanadate

Department of Pharmacology, University of Nevada School of Medicine, Reno, Nevada

Experiments were carried out to examine the effects of protein tyrosine kinase (PTK) and protein tyrosine phosphatase inhibitors on the purinergic contraction of the guinea pig vas deferens. Sodium orthovanadate (SOV) produced a robust increase of the amplitude of contractions evoked by both neurogenic electrical field stimulation and exogenous ATP. This effect of SOV was concentration- and time-dependent, as well as, reversible and reproducible. Genistein, a PTK inhibitor, but not its inactive structural analog daidzein, inhibited the SOV-induced facilitation of the purinergic contraction. Another PTK inhibitor, 2,5-dihydroxycinnamic acid methyl ester, which is structurally unrelated to genistein, also inhibited the facilitation effects of SOV. Although an application of as low as 3 µM of these inhibitors significantly decreased the effect of SOV, other PTK inhibitors, namely, butein, levandustin C, and thyrphostin 23, were less effective even at concentrations of 100 µM. Western blot experiments showed that the facilitation of the purinergic contraction by SOV is associated with a prominent increase in the level of tyrosine phosphorylation of proteins with estimated molecular sizes of 180 and 123 kDa, which was reversed in the presence of genistein. Evidence is also presented that argue against the possibility that inhibition of the Na⁺/K⁺-ATPase or ATPases, responsible for the clearance of ATP is involved in the SOV-induced facilitation of the purinergic contraction. Together, these results suggest that the responsiveness of the smooth muscle of the vas deferens to the actions of ATP is modulated via a previously unidentified mechanism, which may involve protein tyrosine phosphorylation.

Address correspondence to: Dr. Latchezar D. Todorov, Department of Pharmacology, University of Nevada School of Medicine, 1664 N Virginia St., Howard Medical Sciences Bldg. MS 318, Reno, NV 89557-0270. E-mail: todorov{at}med.unr.edu