INTESTINAL RELAXATION PRODUCED BY SYMPATHOMIMETIC AMINES IN THE ISOLATED RABBIT JEJUNUM: SELECTIVE INHIBITION BY ADRENERGIC BLOCKING AGENTS AND BY COLD STORAGE

¹ Department of Pharmacology, Marquette University, School of Medicine, Milwaukee, Wisconsin

The effects of sympathomimetic amines on fresh segments of isolated rabbit jejunum were compared with segments previously stored at 6-8°C for 24 to 72 hr. Small doses of phenylephrine and methoxamine which relaxed the fresh intestine produced little or no response in stored preparations. Storage also produced some decrease in the sensitivity of the intestine to epinephrine and norepinephrine in comparison to isoproterenol. The decrease in sensitivity was greater for epinephrine than for norepinephrine. Storage markedly reduced or abolished the intrinsic activities of methoxamine and phenylephrine, but not those of epinephrine, norepinephrine or isoproterenol. In fresh segments, methoxamine and phenylephrine showed high susceptibility and isoproterenol little or no susceptibility to blockade by dihydroergotamine and phentolamine. The opposite was observed with blockade by pronethalol. Epinephrine and norepinephrine were intermediate in their susceptibility to blockade with alpha and with beta adrenergic blocking agents. Phentolamine, which reduced the relaxation induced by epinephrine and norepinephrine in fresh preparations, failed to do so in stored preparations. Epinephrine, norepinephrine and isoproterenol were found to be equally susceptible to blockade by pronethalol in stored preparations. These results are consistent with the proposal that both alpha and beta receptors subserve the function of intestinal relaxation, and suggest that in vitro storage produces a selective impairment or loss of alpha receptor activity in the rabbit jejunum.