Analysis of the responses of rainbow lizard (Agama agama Linn.). Isolated portal vein to electrical stimulation and to drugs.

The responses of the rainbow lizard (Agama agama Linn.) isolated portal vein to transmural electrical stimulation and to drugs have been examined. The isolated venous muscle preparation strongly contracted in response to exogenously administered adrenaline and noradrenaline (10(-8)-2.5 x 10(-6) M) and also weakly contracted to acetylcholine and 5-hydroxytryptamine (10(-7) - 10(-5) M). The vein relaxed in response to exogenous administrations of isoprenaline (10(-8) - 2.5 x 10(-6) M) and adenosine triphosphate (ATP, 10(-7) - 10(-5) M). Transmural electrical stimulation of the isolated vein at 1-40 Hz induced frequency-dependent contractions of the venous smooth muscle which were inhibited or abolished by tetrodotoxin, alpha-adrenoceptor blockers, adrenergic neurone blocking agents, local anaesthetics, and by pretreatment of rainbow lizards with reserpine. Analysis of these findings shows that the electrical stimulation-evoked contractions of the venous blood vessel resulted from the excitation of post-ganglionic sympathetic neurones. A comparison of the relative potentiation of the venous preparation contractions induced by electrical stimulation, adrenaline and noradrenaline suggested that noradrenaline is the sympathetic transmitter mediating the contractile effects due to electrical excitation. The advantages of this preparation for studying sympathetic nerve-smooth muscle mechanisms and as a pharmacological model for investigating venous blood vessel activity are discussed.