Effects of selective alpha 1- and alpha 2-adrenoceptor agonists on the physiologic properties of rat left atria.

The effects of the selective alpha 1-adrenoceptor agonist, phenylephrine, and the alpha 2-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 catecholamine in rat left atria and the blockade of intra- and extraneuronal uptake of catecholamine, reserpine (10 mg/kg, i.p.) was given to rat 16-18 h before the experiment; cocaine (30 microM) and hydrocortisone (30 microM) were continuously present in the bath. The positive inotropic effect of phenylephrine was competitively antagonized by prazosin with a pA2 value of 9.29. The excitability of rat left atria was not affected by phenylephrine, whereas the automaticity of left atria induced by adrenaline was enhanced and the functional refractory period of left atria was prolonged by phenylephrine. The prolongation of functional refractory period caused by phenylephrine was competitively inhibited by prazosin with a pA2 value of 8.15. B-HT933 was not able to induce any change in the contractility, excitability, automaticity, and functional refractory period of rat left atria under the same conditions. The results indicated that the cardiophysiological effects on rat left atria caused by stimulation of postsynaptic alpha-adrenoceptor was mediated by alpha 1-type receptors while postsynaptic alpha 2-type receptors are not of functional importance on rat left atria.