Non-neural electrical responses of smooth muscle cells of the rabbit basilar artery to electrical field stimulation

In smooth muscle cells of the rabbit basilar artery, field stimulation evoked a depolarizing response which consisted of a fast (1-3 s duration) and a following slow (1-4 min duration) component. The amplitude of these responses increased in an intensity-dependent manner and, when exceeding 10-15 mV, a spike potential was generated. During generation of the slow depolarization, ionic conductances of the membrane were increased. When outward current pulses with long duration (2-3 s) were applied to the smooth muscle using the partition stimulating method, electrotonic potentials and spike potentials were generated. The cessation of the current pulse caused repolarization of the membrane with time constant of 250-350 ms. The depolarizing responses were resistant to tetrodotoxin, sympathetic transmission blocking agents (guanethidine, bretylium, or 6-hydroxydopamine treatment), receptor antagonists for 5-hydroxytryptamine (methysergide), dopamine (haloperidol), ACh (atropine), noradrenaline (phentolamine), ATP (alpha,beta-mATP) or histamine (mepyramine), blockade of synthesis of prostaglandins or thromboxane A2 (indomethacin) or high Mg2+, low Ca2+ solution. Smooth muscle cell membrane of the basilar artery was depolarized by 5-hydroxytryptamine (above 0.1 microM) or histamine (above 10 microM) but not by ACh (up to 100 microM) or noradrenaline (up to 10 microM). The depolarization induced by 5-hydroxytryptamine or histamine was antagonized by methysergide or mepyramine, respectively. Denervation of the vessel by storing in a cold condition (4 degrees C) decreased but did not abolish the depolarizing response. The decrease in amplitude of the depolarizing response during cold storage was attributed to associated depolarization of the smooth muscle membrane. Internal perfusion of the vessel with distilled water abolished generation of the depolarizing response, and this procedure also abolished the endothelium-dependent relaxation induced by ACh during the potassium contraction. The results suggest that the depolarizing response evoked by field stimulation is generated by substances released from non-neural components, possibly from the endothelial cells.