TENSION DEVELOPMENT AND ASSOCIATED CALCIUM INFLUX OF CONTROL AND RESERPINE PRETREATED RABBIT AORTAE IN RESPONSE TO NOREPINEPHRINE, ISOPROTERENOL AND ACETYLCHOLINE
The influence of reserpine (3 mg/kg) pretreatment on phasic and tonic components of the contractile responses of rabbit aorta to maximally effective concentrations of norepinephrine, acetylcholine and isoproterenol and on agonists-induced movements of 45Ca during these responses were studied. After reserpine administration (4 h), there were no histological changes, no changes in extracellular space and no change in tissue water, Na, Ca or Mg. K contents were slightly decreased and the catecholamine content was depleted by 95%. The responses to the 3 agonists were enhanced in the reserpine pretreated tissues. The absence of Ca eliminated the slow tonic component of the responses. Reserpine induced an increase in La3+ resistant 45Ca uptake. The 3 agonists induced an increase in La3+ resistant 45Ca uptake of equal magnitude in both reserpine pretreated and control tissues. La ion affected 45Ca efflux similarly in both reserpine pretreated and untreated tissues. A major portion of the enhanced responsiveness of rabbit aortic strips following reserpine administration is the result of a change in Ca retention in those tissue stores mobilized by these agonists to initiate the fast, phasic component, and is a reflection of an increased agonist intrinsic activity.