Adrenergic receptors of isolated snake atria.

Cardiac adrenergic receptors in snakes were examined using an isolated atria preparation of Naja naja and Ptyas korros. Treatments included an examination of the atrial responses to selective alpha- and beta-adrenergic agonists and antagonists. In both species, both phenylephrine and isoproterenol produced dose-dependent increases in the atrial beating rate and tension. Phenylephrine-induced increases were characterized with a high affinity and low affinity components. These positive chronotropic and inotropic effects produced by phenylephrine and isoproterenol were abolished with propranolol and in the phenylephrine-induced response phentolamine also attenuated the low affinity response and blocked the high affinity response. With catecholamines depletion via 6-OH dopamine or reserpine, the high affinity component in the phenylephrine-induced response was no longer observed. It is concluded that beta-adrenoceptors are the predominant post-synaptic adrenoceptors in snake atria. Stimulatory presynaptic alpha-adrenoceptors for modulating noradrenaline release may also be present.