Effects of K+ channel blockers and nitric oxide on the electrical and contractile activities of smooth muscles of the rabbit main pulmonary artery

Using a sucrose-bridge technique, we studied electrical and mechanical responses of smooth muscle ring strips of the rabbit main pulmonary artery to applications of blockers of voltage-operated (including Ca2+-dependent) K+ channels, tetraethylammonium (TEA) and 4-aminopyridine (4-AP), as well to application of nitric oxide (NO); nitroglycerin (NG) was used as a donor of the latter. All experiments were carried out under conditions of blockade of the adreno- and cholinoreceptors in the preparation. Both TEA and 4-AP evoked dose-dependent effects: depolarization of smooth muscle cells (SMC) and their contraction. Simultaneous addition of TEA and 4-AP to the normal superfusate (Krebs solution) resulted in intensification of depolarization and initiated generation of action potentials (AP); contractions became rather intensive and possessed a tetanic pattern. Addition of NG to TEA- and 4-AP-containing Krebs solution effectively suppressed AP generation and contractions, whereas the depolarization level underwent only mild modifications. These findings show that Ca2+-dependent high-conductance K+ channels (K-Ca channels) and 4-AP-sensitive voltage-operated K+ channels (K-v channels) are involved in the formation of the resting membrane potential (RMP) in SMC of the rabbit main pulmonary artery. The impact of the K-Ca channels is greater than that of the K-v channels. We suppose that the effects of NO on SMC are related to inhibition of the activity of high-threshold voltage-operated L-type Ca2+ channels and, probably, to lowering of the sensitivity of the contractile SMC apparatus to Ca2+.