[Excitation-contraction coupling in the smooth muscle cells of the portal vein as affected by noradrenaline].

Verapamil (10(-5) M) blocked phasic and tonic components of potassium contracture of smooth muscle cells (SMC) of the rat portal vein but failed to block completely noradrenaline, induced contraction of these cells, neither did it suppress the NA-induced contraction of the portal vein SMC in the presence of potassium contracture. These data suggest that NA-induced contraction is activated by external Ca2+ ions entering the SMC through chemosensitive (NA) voltage-independent Ca channels of the membrane. The contraction blocked by verapamil is activated by external Ca2+ ions entering through fast voltage-dependent inactivating Ca channels participating in AP generation, and slow voltage-dependent inactivating Ca channels opened by NA depolarization. The basal tone and contraction evoked by transmitters and physiologically active substances in the SMC of all blood vessels seem to be activated primarily by external Ca2+ entering the SMC via the above mentioned types of Ca channels.