Effects of sympathetic innervation and temperature on the properties of rat heart adrenoceptors

Abstract

1. The pharmacological characteristics of adrenoceptors at different temperatures were assessed on the basis of the effects of various alpha- and beta-adrenoceptor agonists and antagonists on electrically-driven left atria and spontaneously-beating pairs of atria from rats. 2. Phenoxybenzamine (Pbz) potentiated inotropic responses of left atria to noradrenaline (NA) at 31 degrees C, produced significantly less potentiation at 24 degrees C and inhibited responses at 17 degrees C; it had little effect on responses to CaCl2. Both Pbz and phentolamine inhibited responses to phenylephrine more effectively at 17 than at 31 degrees C. N-cyclohexylmethyl-N-ethyl-beta-chloroethylamine hydrochloride (GD-131), a haloalkylamine with negligible alpha-adrenoceptor blocking activity, caused only potentiation of responses to NA at 17 degrees C. 3. The presence of phentolamine during incubation with Pbz eliminated block of responses to NA and revealed a potentiation that was equivalent at all three temperatures tested. Phentolamine did not alter the block of responses to 5-hydroxytryptamine by Pbz. Protection of alpha-adrenoceptors by phentolamine during exposure to [3H]-Pbz significantly decreased the amount of label bound to the myocardium at 17 degrees C, but did not alter binding at 31 degrees C. 4. Inhibition of responses to NA by propranolol decreased with temperature, and the magnitude of the change increased with the concentration of propranolol. Compared to 31 degrees C, the effect of the highest concentration of propranolol. (4.0 micronM) was significantly decreased at 24 degrees C, and the effects of all except the lowest concentration (0.04 micronM) were significantly decreased at 17 degrees C. 5. The potency of isoprenaline decreased and that of phenylephrine increased at low temperatures, and their potency ratio was much lower at 17 than at 31 degrees C for both the inotropic and chronotropic responses of spontaneously-beating atria. However, the ratio was unaffected by temperature in electrically-driven left atria. A similar difference between spontaneously-beating and driven preparations is apparent in the data of other workers, but its basis is not clear. 6. Atria from rats pretreated with 6-hydroxydopamine (6-OHDA) were sensitized to the effects of NA, and there was no increase in alpha-adrenoceptor properties at low temperatures. Little alpha-adrenoceptor activity could be demonstrated in chemically denervated atria at any temperature, 6-OHDA pretreatment did not alter the binding of [3H]-Pbz at 31 degrees C, but decreased it significantly at 17 degrees C. Pretreatment with reserpine caused some sensitization, but not significantly after the characteristics of the adrenoceptors or their responses to temperature. 7. It is concluded that the adrenoceptors of atria are affected by temperature in much the same way as those of frog hearts, although the transition from beta- to alpha-adrenoceptor properties may begin at a slightly higher temperature...