[Nature of electromechanical connections in the smooth muscle cells of the pulmonary artery].

Depolarization of the rabbit pulmonary artery smooth muscle cells over critical level (5-7 mV) by outward current or high K+ solution application produced contraction proportional to the depolarization. High K+ solution produced an initial spike on the rising phase of depolarization followed by subsequent phasic contraction amplitude of which was lower than that of the tonic one. Anelectrotonic repolarization of smooth muscle cells by means of inward current in high K+ solution was usually followed by spontaneous AP generation and subsequent phasic contractions which were summed and increased total contractile tension. If the spontaneous APs were not generated anelectrotonic repolarization was followed by relaxation of the smooth muscle proportional to the extent of repolarization. AP in this case could be evoked by electrical stimulation. Experiments with Ca--free solution and application of Ca and K-channel blockers suggest the existence of two types of voltage--dependent Ca--channels in the membrane of these smooth muscle cells: fast inactivated responsible for AP generation and phasic contraction, and slow noninactivated responsible for tonic contractions, dependent on the transmembrane potential. In physiological conditions, a functional role of the fast Ca channels is limited due to early activation of K-conductance which is comparatively large in these smooth muscle cells and suppresses AP generation.