EFFECTS OF 3, 4‐DIHYDRO‐8‐(2‐HYDROXY‐3‐ISOPROPYLAMINOPROPOXY)‐3‐NITROXY‐2H‐1‐BENZOPYRAN (K‐351) ON SMOOTH MUSCLE CELLS AND NEUROMUSCULAR TRANSMISSION IN THE CANINE MESENTERIC ARTERY

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Abstract
The effects of K‐351 on the electrical and mechanical responses were investigated in the canine mesenteric artery by isometric tension recording and the use of intracellular microelectrodes. The results were compared to the responses observed with other α‐adrenoceptor blocking agents. K‐351 (> 3 × 10−7m) consistently inhibited the contraction evoked by perivascular nerve stimulation; however, K‐351 had no effect on the contraction evoked by direct muscle stimulation, after pretreatment with 3 × 10−7m tetrodotoxin. Phentolamine enhanced and prazosin had no effect on the amplitude of contraction evoked by perivascular nerve stimulation at a high frequency (over 1.0 Hz). Pretreatment with phentolamine inhibited the contraction evoked by lower frequencies of perivascular nerve stimulation (below 0.5 Hz). The potency for the inhibition of the response to perivascular nerve stimulation was in the order of K‐351 > phentolamine > prazosin, while the contractions induced by exogenously applied noradrenaline (5 × 10−7m) were inhibited in the order: prazosin > phentolamine > K‐351. K‐351 (< 3 × 10−5m) did not modify the resting membrane potential or the membrane resistance, as estimated from the change in the amplitude of electrotonic potentials in the smooth muscle cell membranes. K‐351 (> 3 × 10−7m) inhibited the amplitude of the first e.j.p. and e.j.ps after completion of the facilitation process following stimulation at frequencies over 0.25 Hz. K‐351 (< 3 × 10−5m) did not modify the compound action potentials recorded from peripheral nerve bundles distributed on the mesenteric artery. Phentolamine (> 1 × 10−8 m) inhibited the first e.j.p. but this agent either inhibited or enhanced the amplitude of e.j.p. after completion of the facilitation process produced by repetitive stimulation below or over 1.0 Hz stimulus frequencies, respectively. Prazosin (1 × 10−6m) had no effect on e.j.ps evoked by perivascular nerve stimulation, at any stimulus frequency applied. These results indicate that K‐351 inhibits the extra‐junctional adrenoceptor with a slightly weaker potency than prazosin or phentolamine, but that this agent has a potent action as an intra‐junctional adrenoceptor blocking agent. Phentolamine acts mainly on the extra‐junctional adrenoceptors and also has weak actions on intra‐junctional adrenoceptors, as a blocking agent. This agent also inhibits the presynaptic α2‐adrenoceptor. Prazosin inhibits only the extra‐junctional α1‐adrenoceptor.

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