Objective:Spontaneously hypertensive rat (SHR) myocardium is known to exhibit reduced β-adrenergic agonist-stimulated adenylyl cyclase activity. As G-proteins play a pivotal role in transducing information from the receptor to adenylyl cyclase, a study was undertaken to assess whether changes in G-protein expression and functioning could explain thisDesign:Studies were performed in plasma membrane homogenates from vascular tissue (myocardium) from 11-week-old SHR (weighing 255 g) and control rats from both Wistar-Kyoto (WKY) and Wistar strains. Similar studies were performed in non-vascular tissue (renal cortical plasma membranes) to assess whether any observed changes are part of a more widely distributed membrane defectMethods:G-protein function was inferred from studies of adenylyl cyclase activity and levels of G-protein subunits (Gsα, Giα1, Giα2, Giα3, Goαand β) were assessed by immunoblottingResults:Differences in adenylyl cyclase activity were seen in SHR compared with WKY rat myocardium: in WKY rats adenylyl cyclase activity was greater than in SHR under forskolin-stimulated conditions and in the presence of fluoride and several ligands which couple to the catalytic unit of adenylyl cyclase via Gs, including the receptor-linked species prostaglandin E1glucagon and isoproterenol. However, with the exception of forskolin-stimulated activity, which in SHR was greater than in Wistar rats, SHR myocardial adenylyl cyclase activities were similar to those in Wistar rat membranes. Immunoblotting studies showed similar levels of G-protein subunits in all three strains. Studies of renal cortical plasma membranes failed to identify any differences in adenylyl cyclase activity or in G-protein subunit levelsConclusions:SHR, WKY rats and Wistar rats exhibit differences in myocardial adenylyl cyclase activity which are not seen in renal cortical plasma membranes. These are not related to hypertension or to differences in G-protein levels, and probably reflect strain differences in Gsfunction