Autonomic receptor interactions in isolated cardiac myocytes from hypertensive rats

Isolated ventricular myocytes from adult (16 to 20 weeks) spontaneously hypertensive (SHR) and normotensive (WKY) rats were utilized to examine adrenergic and cholinergic receptor expression and interaction. Binding assays were performed using quinuclidinyl benzilate (QNB) and iodocyanopindolol (ICYP) for cholinergic and beta-adrenergic receptors, respectively. In addition, cAMP was measured as an index of adrenergic-cholinergic control of adenylate cyclase. Data from radioligand binding experiments indicated that muscarinic cholinergic receptors were depressed (22%) in SHR myocytes, while beta-adrenergic receptor density was comparable to that of WKY myocytes. Heterologous receptor modulation in isolated myocytes as assessed by displacement analysis with and without guanosine 5'-triphosphate (GTP), showed that carbachol displacement of QNB was shifted five fold to the right in the presence of GTP and that the beta-adrenergic agonist isoproterenol did not prevent the GTP-mediated binding alteration. In contrast, carbachol modulated the GTP-shift of ICYP displacement by isoproterenol and these effects were comparable in both WKY and SHR myocytes. Furthermore, the ability of carbachol to blunt the stimulation of adenylate cyclase by isoproterenol was also comparable in myocytes isolated from adult SHR and control animals. Thus, the observed decrement in muscarinic cholinergic receptor expression did not alter adrenergic-cholinergic interactions as assessed by displacement assays using guanine nucleotides, or the control of cAMP levels. In addition, isolated myocytes provide a useful system for analyzing receptor expression and regulation and how these parameters may be altered in the hypertensive heart.