Factors controlling myogenic activity in smooth muscle

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Abstract
The possible roles of Na and Ca in myogenic spontaneous activity are described. Na ions influence the membrane potential by the Na concentration gradient across the membrane, by the relative membrane permeability to Na and by an electrogenic factor involving the Na-K pump. Another role of Na is its contiibution to the regulation of the Ca distribution across the membrane, probably through Na-Ca exchange and by affecting Ca permeability. Although these functions of Na are important for the generation of spontaneous activity, the primary role of Ca is emphasized since spontaneous activity can be produced in Na-free, Ca containing solution. It is postulated that the internal surface of the membrane has negative sites, to which Ca can be bound, and that the K permeability is increased when the amount of bound Ca at this site is increased. The K permeability would then be reduced when Ca is extruded by an active Ca pump, and this may be the mechanism underlying the pacemaker potential.

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