α1D-Adrenoceptors Do Not Contribute to Phosphoinositide Hydrolysis in Adult Rat Cardiac Myocytes

We have used the α1D-adrenoceptor selective antagonist, BMY 7378, to investigate the presence of α1D-adrenoceptor subtype in adult rat heart by radioligand binding assays. We also determined the role of this subtype in stimulating phosphoinositide (PI) hydrolysis in adult rat cardiac myocytes. BMY 7378 inhibited [3H]prazosin binding to cardiac membranes in a biphasic mode with a pKi of 9.19 ± 0.26 for high affinity sites and 6.64 ± 0.09 for low affinity sites. The inhibition of the adrenaline-induced stimulation of PI hydrolysis by BMY 7378 fitted a one-site model and the calculated pKb value (6.92 ± 0.28) was consistent with the involvement of α1A and α1B adrenoceptors. In addition, BMY 7378, at concentrations up to 100 nM, did not significantly affect the concentration-response curves for the adrenaline-induced stimulation of PI hydrolysis. Taken together, these data suggest that α1D-adrenoceptors are expressed in adult rat heart but this subtype is not involved in the adrenaline-induced stimulation of PI hydrolysis.