Vasopressin modulates the spontaneous electrical activity in aortic cells (line A7r5) by acting on three different types of ionic channels.

A7r5 smooth muscle (aorta) cells have a spontaneous electrical activity. Application of vasopressin produces a hyperpolarization accompanied by an interruption of the spontaneous activity, which is followed by a depolarization associated with a recovery of the spiking activity. Vasopressin action is produced by an action of the peptide on three different types of ionic channels. Vasopressin activates a Ca2+-sensitive K+ conductance, presumably by producing inositol 1,4,5-trisphosphate intracellularly and liberating Ca2+ from internal stores. This activation is transient (0.5-4 min) and is related to the vasopressin-induced hyperpolarization. Intracellular perfusion of inositol trisphosphate triggers by itself a transient K+ current and prevents subsequent activation by vasopressin. Vasopressin inhibits an L-type Ca2+ channel through both protein kinase C activation and a [Ca2+]i-dependent inactivation mechanism triggered by inositol trisphosphate production. The addition of the activation of a Ca2+-sensitive K+ channel and of the inhibition of a voltage-sensitive Ca2+ channel is responsible for the transient blockade of the spontaneous activity. Vasopressin also provokes the activation of an inward current (2-20 min) due to a nonselective channel able to transfer Ca2+, Na+, K+, and Cs+ across the membrane. This effect of the peptide is associated with the depolarization following the hyperpolarization phase.