Effects of α_1-and α_2-adrenoceptor agonist on the physiological properties of rat left atria

The effects of selective α1-adrenoceptor agonist, phenylephrine and α2-adrenoceptor agonist, B-HT933 on the contractility, excitability, automaticity and functional refractory period in the isolated rat left atria, were investigated.For the depletion of catechol-amine in rat left atria and the blockade of intra- and extra-neuronal uptake of catecholamine, reserpine (10mg/kg,ip)was given to rat 16-18 h before the experiment. Cocaine(30μM) and hydrocortisone ( 30 μM) were continuously present in the bath.Positive inotropic effect of phenylephrine was competitively antagonized by prazosin (0.1 μM) with pA2 value of 9.29. Verapamil (0.5μM) also inhibited the positive inotropic effect of phenylephrine but in non-competitive manner. The excitability of rat left atria was not affected by phenylephrine, whereas the automa-ticitv of left atria induced by adrenaline was enhanced and the func-tional refractory period of left atria was prolonged by phenylephrine. The prolongation of functional refractory Period (from 97±8 ms of control to 130±13ms) caused by phenylephrine was competitively inhibited by pra-zosin(0.1 μM) with pA2value of 8.15.B-HT933 was not able to induce any change of the contractility, excitability, automaticity and functional refractory period of rat left atria in the same conditions.The results indicated that the cardiophysiological effects of rat left atria caused by stimulation of post-synaptic α-adrenoceptor was mediated by α1-type receptors while postsynap-tic α2-type receptors are not of functional importance on rat left atria.