| Abstract: | Beta adrenoceptor density, measured by radioligand binding techniques, is reportedly much higher in atrial than in ventricular myocardium of patients with mitral valve disease. In the present study adenylate cyclase activity, both basal and in response to beta adrenergic agonists and sodium fluoride, in biopsy preparations from these same patients was significantly lower in atrial than in ventricular tissue when stimulated with either isoproterenol, noradrenaline, isoproterenol combined with terbutaline, or sodium fluoride. Terbutaline stimulated and basal adenylate cyclase activity was not significantly different in the two cardiac regions. The ratios of receptor density to isoproterenol stimulated and to sodium fluoride stimulated adenylate cyclase activity were 4-5 times higher in atrial than in ventricular biopsy specimens. Thus ventricular beta receptors, although present in comparatively low concentrations, are coupled to considerably more catalytic moieties of the receptor-adenylate cyclase complex than their atrial counterparts. The reason for this is probably a relative lack of coupling proteins (N components) in atrial tissue. A weak positive correlation between receptor density and isoproterenol stimulated adenylate cyclase activity was found in atrial but not in ventricular tissue. This may indicate individual variation in receptor-adenylate cyclase coupling. Furthermore, no correlation was found between atrial and ventricular values for any variable. One reason for this may be the different haemodynamic stresses in the two cardiac chambers. |
