Venous smooth muscle in hypertension. Enhanced contractility of portal veins from spontaneously hypertensive rats.

In isolated bath studies smooth muscle from the rat portal vein was evaluated for its reactivity and contractility, and the whole vessel wall was evaluated for its extensibility. Smooth muscle from the spontaneously hypertensive rat (SHR) had the following characteristics when compared with that from normotensive controls: (1) Spontaneous phasic contractions were more frequent and developed more tension; (2) threshold concentrations for responses to prostaglandins A-2 and B-2 were lower, but those for responses to epinephrine, norepinephrine, KCl, BaCl-2, and SrCl-2 were similar; (3) high concentrations of calcium had a less depressant action on the responses to the prostaglandins but not on the responses to the other agonists; (4) maximal contractile tensions to all agonists were greater; and (5) passive extensibility was less. These differences, because they are in the venous system, cannot be secondary to the increase in wall stress of arterial hypertension. The decreased passive extensibility in this vein in SHR creates a stiffer framework on which the active contractile process is able to develop greater tension. If this increase in active tension is generalized to all veins, it could be responsible for a decrease in venous capacity which increases the rate of venous return and, hence, increases cardiac output.