Differential sensitization of cardiac actions of adenosine in rats after chronic theophylline treatment.

To determine the effect of chronic adenosine receptor blockade on atrial responsiveness, we administered theophylline to rats in their drinking water (0.6 mg/ml) for 2 wk. Inotropic and chronotropic responses to the adenosine receptor agonists N6-cyclopentyladenosine (CPA) and 5'-(N-ethylcarboxamido)-adenosine (NECA) were then measured in isolated atria from treated and control animals. The indirect (antiadrenergic) actions of CPA and NECA on force and rate, measured during beta-adrenergic receptor stimulation by isoproterenol, were markedly sensitized (2- to 10-fold reductions in the agonist concentration needed to obtain a half-maximal response) after theophylline. The sensitization was homologous because inotropic and chronotropic responses to carbachol were not affected by theophylline. The direct negative inotropic and chronotropic actions of CPA and NECA, measured without concomitant beta-adrenergic stimulation, were not sensitized after theophylline. The number of atrial A1-receptors, measured by antagonist radioligand binding (maximum specific binding at saturation), was increased by 22% in theophylline-treated rats [66.2 +/- 3.4 vs. 54.3 +/- 1.9 (control) fmol/mg protein, P < 0.05]. Competition binding indicated that the fraction of coupled (high-affinity) receptors was unchanged. The number of ventricular A1-receptors was increased to a similar extent without any change in coupling. Thus chronic dietary theophylline upregulated cardiac A1-adenosine receptors without changing coupling state or affinity and sensitized rat atria to the indirect, antiadrenergic, inhibitory inotropic and chronotropic actions of adenosine receptor agonists.