Effects of Cyclopentyladenosine on Isoproterenol Response in Adult and Senescent Cardiac Tissue from Fischer 344 Rats.

To characterize age-related changes in beta-adrenergic responsiveness and to test the hypothesis that an increase in the effects of adenosine contribute to impaired beta-adrenergic responsiveness, Fischer 344 rat right atria (RA), left atria (LA), and left ventricular trabeculae carnae were exposed to the beta-receptor agonist isoproterenol (ISO), followed by four doses of the selective adenosine A(1) receptor agonist cyclopentyladenosine (CPA). Spontaneous contractile rates of adult RA were inhibited more than senescent RA by CPA. Contractility (+dF/dt) of adult LA was reduced more than senescent LA by CPA. Left trabeculae carnae tissue responded weakly to CPA, but senescent tissue was less responsive than adult tissue. Senescent atrial A(1) receptor density was 56% greater than in adult tissue, whereas the density in senescent ventricles was 39% lower than in adult tissue. No significant difference in antagonist affinities (K(d)) of A(1) receptor was observed between adult and senescent atria. In addition, agonist competition curves indicated a significant increase in senescent atrial and a decrease in senescent ventricular tissue in the affinity of agonist for high-affinity A(1) receptors with no difference in dissociation constant (K(i)). No significant age-related differences in atrial or ventricular tissues occurred in either the antagonist affinity (K(d)) or density (B(max)) of the beta-adrenergic receptors. CPA was found to inhibit ISO-stimulated adenylate cyclase activity more in senescent than in adult atrial and ventricular membrane preparations. We conclude that age-related differences in functional response to ISO and CPA, A(1) receptor density, and ISO-stimulated adenylate cyclase activity differ in atrial and ventricular myocardium.