Effect of reserpine pretreatment on mechanical responsiveness and [125I]Iodohydroxybenzylpindolol binding sites in the guinea-pig right atrium.

The inotropic and chronotropic responses of the guinea-pig right atrium to several pharmacologic agents were measured after acute (0.1 mg/kg/day x 1) and chronic (0.1 mg/kg/day x 7) reserpine administration. A small increase in the sensitivity of the pacemaker to isoproterenol occurred after acute reserpine treatment which was followed by a much greater change in sensitivity to the beta agonist when pretreatment was extended for 7 days. Chronotropic responsiveness to calcium, histamine and pilocarpine was not altered by reserpine pretreatment. The acute administration of reserpine resulted in a slight inotropic supersensitivity of paced right atria to isoproterenol, calcium and histamine. Pretreatment for 7 days produced an additional increase in inotropic sensitivity to isoproterenol but did not affect contractile responses to the other agents. The catecholamine-specific nature of the supersensitivity induced by chronic reserpine treatment suggested that a change in the number and/or affinity of beta adrenergic receptors was involved. The radiolabeled beta adrenoceptor antagonist [125]iodohydroxybenzylpindolol (I-HYP) was used to test this hypothesis. Preliminary experiments revealed the presence of a single class of noninteracting (nH = 0.99), high affinity (Kd = 100 pM) binding sites which exhibited stereospecificity and saturability (47.2 fmol/mg of protein). The agonist potency series for the inhibition of I-HYP binding was identical to the series for mediating mechanical responses. Taken collectively this information suggests that the high affinity I-HYP binding site in the guinea-pig right atrium represents the beta adrenergic receptor. As determined by Scatchard analyses, neither acute (1-day) nor chronic (7-day) low-dose (0.1 mg/kg/day) reserpine administration altered the number or affinity of I-HYP binding sites. It is concluded that changes in beta receptor characteristics are not responsible for reserpine-induced supersensitivity in this tissue.