| Abstract: | 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. |
