Genetic influences on agonist binding to cardiac beta-receptors

Regulation of beta-adrenoceptor-agonist function in the Maudsley Reactive (MR/Har) and the Maudsley Non-Reactive (MNRA/Har) rat strains was assessed by comparison of isoproterenol competition for [125I]iodocyanopindolol (ICYP) binding sites in crude left ventricular homogenate preparations. Non-linear, least-squares analysis of isoproterenol competition for ICYP binding in the absence of guanine nucleotide revealed different proportions of high- and low-affinity receptors in the two strains; MR/Har rats (59 +/- 3.3%) had a significantly greater proportion of receptors in the high-affinity state than the MNRA/Har rats (41 +/- 4.5%). Addition of the non-hydrolyzable guanine nucleotide analog guanylylimidodiphosphate (Gpp(NH)p) converted receptors to the low-affinity state. Analysis of Gpp(NH)p concentration-response curves in left ventricular homogenates of the two strains revealed that the MR/Har strain had a significantly (P less than 0.02) lower EC50 for guanyl nucleotide inhibition of isoproterenol competition for ICYP binding than the MNRA/Har. Confirming previous experimental results, a significantly (P less than 0.04) greater density of ventricular beta-receptors was found in MR/Har rats (13.16 +/- 0.92 fmol/mg protein) than in MNRA/Har rats (10.81 +/- 0.63 fmol/mg protein). Left ventricular catecholamine levels were found to be correlated inversely with beta-adrenoceptor density in the two strains; norepinephrine (NE) and epinephrine (EPI) concentrations (ng/mg protein) in left ventricle were 12.19 +/- 0.94 for NE and 0.165 +/- 0.038 for EPI in MNRA/Har, and 8.73 +/- 0.95 and 0.018 +/- 0.018, respectively, in MR/Har. All other parameters of agonist interactions with the cardiac beta-adrenoceptor for the MR/Har and MNRA/Har rat strains were similar [the IC50 for displacement of ICYP binding by isoproterenol, the accompanying Hill coefficients in the Gpp(NH)p present and absent condition, the Kd of the high- and low-affinity states in the absence of Gpp(NH)p, and the Kd of the uniform low-affinity state in the presence of Gpp(NH)p]. We hypothesize that the strain-dependent differences in high-affinity state formation reported here may account for some of the in vivo differences in cardiovascular function previously demonstrated in the Maudsley rats.