RADIOLIGAND BINDING-STUDIES OF ALPHA(1)-ADRENOCEPTOR SUBTYPES IN RAT-HEART

1 In order to characterize the alpha(1)-adrenoceptor subtypes mediating positive inotropic effects of adrenaline (in the presence of propranolol) in rat right ventricular strips and the Ca2+ sources used to elicit them, we have used radioligand binding to identify the alpha(1)-adrenoceptor subtypes present in rat heart and the alpha(1)-adrenoceptor affinity and subtype-selectivity of various pharmacological tools. 2 Amitryptiline, mianserin, trimipramine, oxaprotiline, clonidine, chloroethyiclonidine, phenoxybenzamine, BE 2254 and 8-OH-DPAT competed for [H-3]-prazosin binding in rat heart, vas deferens, liver, spleen, cerebral cortex and hippocampus but none of them displayed detectable alpha(1)-adrenoceptor subtype-selectivity; nitrendipine did not compete for [H-3]-prazosin binding in concentrations up to 5 mu mol 1(-1). 3 The alpha(1A)-adrenoceptor-selective, 5-methyl-urapidil, (+)-niguldipine, and to a lesser extent (-)niguldipine competed for [H-3]-prazosin binding in rat heart, vas deferens, cerebral cortex and hippocampus with shallow and biphasic curves; analysis of these curves demonstrated that rat heart contains alpha(1A)- and alpha(1B)-adrenoceptors in a 20:80 ratio. 4 Treatment of rat right ventricular strips with 100 mu mol 1(-1) chloroethylclonidine for 30 min at 30 degrees C followed by 60min washout reduced the number of alpha(1)-adrenoceptors, as assessed by [H-3]-prazosin saturation experiments, by 74%. Treatment with 100 mu mol 1(-1) CdCl2 did not affect number or affinity of cardiac alpha(1)-adrenoceptors and combined treatment with chloroethylclonidine and CdCl2 reduced alpha(1)-adrenoceptor number by 90%. 5 Treatment of rat right ventricular strips with chloroethylclonidine steepened 5-methyl-urapidil competition curves and increased the relative contribution of c alpha(1A)-adrenoceptors from 26 to 89%. Treatment with CdCl2 did not affect 5-methyl-urapidil competition curves and combined treatment with chloroethylclonidine and CdCl2 increased the relative contribution of alpha(1A)-adrenoceptors to 66%. 6 We conclude that rat heart contains alpha(1A)- and alpha(1B)-adrenoceptors in a 20:80% ratio. Treatment with chloroethylclonidine reduces alpha(1B)-adrenoceptor number by 96% but has only minor effects on alpha(1A)-adrenoceptor density. Treatment with CdCl2 does not affect the number of either alpha(1)-adrenoceptor subtype.