 1D -Adrenoceptors Do Not Contribute to Phosphoinositide Hydrolysis in Adult Rat Cardiac Myocytes Stelios Seraskeris,* Catherine Gaitanaki,† and Antigone Lazou* ,1 *Laboratory of Animal Physiology, Department of Zoology, School of Biology, Aristotle University of Thessaloniki, Thessaloniki 54006; and †Department of Animal and Human Physiology, School of Biology, Faculty of Sciences, University of Athens, Athens 15784, Greece Received March 12, 2001; published online July 9, 2001 We have used the  1D -adrenoceptor selective antag- onist, BMY 7378, to investigate the presence of  1D - adrenoceptor subtype in adult rat heart by radioli- gand binding assays. We also determined the role of this subtype in stimulating phosphoinositide (PI) hy- drolysis in adult rat cardiac myocytes. BMY 7378 in- hibited [ 3 H]prazosin binding to cardiac membranes in a biphasic mode with a pK i 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 pK b value (6.92  0.28) was consistent with the involvement of  1A and  1B adrenoceptors. In addi- tion, BMY 7378, at concentrations up to 100 nM, did not significantly affect the concentration-response curves for the adrenaline-induced stimulation of PI hydroly- sis. Taken together, these data suggest that  1D -adre- noceptors are expressed in adult rat heart but this subtype is not involved in the adrenaline-induced stimulation of PI hydrolysis. © 2001 Academic Press Key Words:  1D -adrenoceptor subtype; phosphoinosi- tide hydrolysis; BMY 7378; adult rat cardiac myocytes. In the heart, activation of  1 -adrenoceptors has a number of physiological effects. These include rapid regulation of contractile activity through changes in chronotropy and inotropy and long-term maintenance of cardiac function through regulation of gene expres- sion and cell growth (1–3).  1 -Adrenergic receptors belong to the larger family of G q/11 -protein coupled receptors, which initiate signals by activating phospholipase C-dependent hydrolysis of membrane phosphoinositides (PI) in almost all tissues where this effect has been examined (3–5). Recent studies have shown that other signaling pathways can been activated upon  1 -adrenoceptor stimulation such as Ca 2+ influx through voltage-dependent and indepen- dent Ca 2+ channels, arachidonic acid release, and phos- pholipase D activation (2–5). Pharmacologically distinct  1 -adrenergic receptor subtypes have been described and molecular cloning and expression of the cDNA for three  1 -subtypes, namely  1A ,  1B , and  1D , have been reported (2, 6, 7). At the RNA level, all three subtypes appear to be present in the heart (7–10). At the protein level, both the  1A - and  1B -adrenoceptor subtypes have been re- ported to be present in cardiac tissue, using selective receptor antagonists (11–14). However, there is a con- troversy concerning the existence of a  1D -adrenoceptor binding site and its functional role. It has been argued that  1D -adrenoceptors are not expressed at the protein level in the myocardium and in many other rat tissues where their mRNA has been described abundantly and they cannot be detected by competition radioligand binding studies (10, 15, 16). On the other hand, a recent study demonstrated the existence of  1D -adreno- ceptors at the mRNA and protein level in rat heart but their functional importance in mediating the inotropic response to noradrenaline remained unclear (17). How- ever,  1D -adrenoceptors have been detected in rabbit myocardium and have been found to contribute to the  1 -adrenoceptor mediated regulation of contractile force in this tissue (18). The physiological rationale for multiple  1 -adreno- ceptor subtypes is largely unclear. One possibility is that the subtypes couple to intracellular signaling pathways in qualitatively and/or quantitatively dis- tinct manners. To this end, it has been demonstrated that in neonatal cardiac myocytes  1A -adrenoceptors 1 To whom correspondence and reprint requests should be ad- dressed. Fax: +30 31 998331. E-mail: lazou@bio.auth.gr. 0003-9861/01 $35.00 117 Copyright © 2001 by Academic Press All rights of reproduction in any form reserved. Archives of Biochemistry and Biophysics Vol. 392, No. 1, August 1, pp. 117–122, 2001 doi:10.1006/abbi.2001.2424, available online at http://www.idealibrary.com on