Effects of phthalazinol on smooth muscle cells of the porcine coronary artery and on neuromuscular junction on the guinea-pig vas deferens

Summary

In smooth muscle cells of the porcine coronary artery, phthalazinol (≦10⁻⁵ M) did not modify the membrane potential and the membrane resistance. At a concentration of 10⁻⁴ M or higher, it hyperpolarized the membrane, reduced the membrane resistance and enhanced the rectifying property of the membrane. At the concentration of 10⁻⁵ M, phthalazinol raised the threshold for the induction of a contraction and suppressed nonselectively the amplitude of the contraction evoked by application of high [K]₀, acetylcholine or electrical depolarization of the membrane. Phthalazinol (10⁻⁵–10⁻⁴ M) did not modify the membrane properties of smooth muscle cells from the guinea-pig vas deferens. However, it suppressed the amplitude of the excitatory junction potentials and the facilitation phenomenon produced by repetitive stimulation at various rates. The action potential recorded from the adrenergic nerves was not affected in the presence of phthalazinol (10⁻⁵ and 10⁻⁴ M). The mean amplitude of the miniature excitatory junction potentials (m.e.j.p.s.) was not affected by treatment with phthalazinol (10⁻⁵–10⁻⁴ M), but the rate of which of m.e.j.p.s. appeared was reduced by phthalazinol (>5×10⁻⁵ M). These results indicate that the vasodilator property of phthalazinol may result from a suppression of Ca-mobilization in both the smooth muscle cells and the adrenergic nerve terminals. The former affects the mechanical response directly and the latter leads to an inhibition of noradrenaline release.