assessed and controlled; second, to define the effects of this intervention on myocardial energetics in terms of any changes dependent on or independent ofalterations ofthese determinants of MVO2; third, to define any change in myocardial sensitivity to catecholamine action after the withdrawal of long-term (3-adrenoceptor blockade; fourth, to determine whether chronic f3-adrenoceptor blockade alters the number, affinity, or biochemical activity of this autonomic receptor.
MethodsFour separate series ofexperiments, each employing paired sets ofcontrol and chronically ,8-blocked cats, comprise the data base for this study. In the first series, mechanical and metabolic behavior of myocardium isolated from these two sets of cats was measured. In the second series, the specific relationship of any differences found between these two sets of animals to changes in the activity of the cardiac fl-adrenoceptor was examined. In the third series, the effect ofchronic fl-adrenoceptor blockade on fl-adrenoceptor number, affinity, and affinity state was assayed. In the fourth series, the effect of chronic fl-adrenoceptor blockade on the major immediate U-mediated biochemical response, cyclic AMP generation, was determined.
Experimental animalsThe animals used were 1.7-3.6-kg adult cats of random sex. The cats in the control groups received normal saline (I cm3/kg i.p.) twice daily for 14 d prior to study. The chronically fl-blocked cats were prepared and characterized as follows. First, their baseline responsiveness to (3-adre-