Thermodependence of basal and stimulated cardiac adenylate cyclase activity in normotensive and spontaneously hypertensive rats.

The cardiac adenylate cyclase activity from normotensive and spontaneously hypertensive (SHR) rats was studied as a function of temperature between 17 degrees and 37 degrees C. Arrhenius plots of adenylate cyclase activity displayed a break around 31 degrees C when tested under basal conditions or in the presence of GTP but were linearized after activation with p[NH]ppG, NaF, secretin, glucagon or isoproterenol. The energy of activation of adenylate cyclase activity in the presence of GTP (9.5 +/- 0.5 kcal/mol) was significantly lower than in the presence of p[NH]ppG (17.7 +/- 0.8 kcal/mol). A hormone was without effect on the energy of activation observed with either GTP or p[NH]ppG but the simultaneous presence of hormone and nucleotide increased markedly the activity of the enzyme. The energies of activation were analyzed in terms of variation of enthalpy and entropy and discussed in relation with the process of activation and coupling of the guanine nucleotide regulatory protein. These thermodynamic characteristics were similar in cardiac membranes from normotensive and spontaneously hypertensive rats, suggesting that the impairment of hormone-stimulated adenylate cyclase activity observed in the heart membranes of hypertensive rats was not a consequence of a defect in the activation process of the enzyme.