The operation of autoregulatory feedback loops in noradrenergic transmission to cardiovascular effector tissues

Acetylcholine release from rat atria can be regulated through an alpha 1-adrenergic receptor

Isolated superfused rat atria release [3H]acetylcholine when depolarized with 57 mM potassium. The depolarization-induced overflow of [3H]acetylcholine is markedly inhibited by micromolar concentrations of epinephrine and norepinephrine. The alpha 1-selective adrenergic agonist methoxamine also inhibits tritium overflow, but the alpha 2-selective adrenergic agonist clonidine and the beta-adrenergic agonist isoproterenol do not. Prazosin, an selective alpha 1-adrenergic antagonist, blocks adrenergic inhibition of [3H]acetylcholine overflow with a Ki of approximately 0.4 nM. Yohimbine has approximately one-hundredth the potency of prazosin for blocking adrenergic inhibition of [3H]acetylcholine overflow. [3H]Norepinephrine overflow from isolated rat atria is also inhibited by norepinephrine, but this effect is antagonized by yohimbine and not by prazosin. We suggest that the release of acetylcholine from cardiac parasympathetic neurons can be regulated through an alpha 1-adrenergic receptor, and that this mechanism may underly, at least in part, the relative lack of effects of prazosin on heart rate.

Alpha 1- and alpha 2-adrenoceptor binding in the Dahl rat model of hypertension

Dahl sensitive rats on a high salt diet (DSH group) developed significant elevations in blood pressure (BP). Sensitive rats maintained on a low salt diet (DSL group) and Dahl resistant rats on a high or low salt diet (DRH and DRL groups, respectively) remained normotensive. The DSH and DRH groups displayed a lower density of alpha 2-adrenoceptors (as measured with [3H]-clonidine) in the cerebral cortex than normotensive DSL and DRL groups. In contrast, the density of alpha 2-adrenoceptors in the medulla was significantly lower in the DSH group than the DSL group, but significantly higher in the DRH group compared to the DRL group. The density of alpha 1-adrenoceptors (as measured with [3H]-WB4101) in the hypothalamus was lower in the DSH group than the DSL group but greater in the DRH group than the DRL group. The results suggest that the sensitive and resistant lines can be distinguished by the density of alpha 1- adrenoceptors in the hypothalamus and medulla, respectively. The interactive effects of dietary NaCl and susceptibility to hypertension on adrenoceptors lend further support to the hypothesis that the genetic predisposition to hypertension is associated with a disruption in central adrenergic activity.

Differences between the prejunctional effects of phenylephrine and clonidine in guinea-pig isolated atria

The prejunctional effects of clonidine and phenylephrine were studied in guinea-pig isolated atria by means of field stimulation of the sympathetic nerve terminals during the cardiac refractory period, in the presence of 1 microM atropine. Clonidine (10-100 nM) produced a dose-dependent decrease in the stimulus-inotropic response curve; the IC50 for clonidine was increased about 70 times by the pretreatment of the preparations with 1 microM yohimbine. The effect of clonidine was not modified by 0.5 microM prazosin. Unlike clonidine, phenylephrine (1-10 microM) induced a statistically insignificant increase in the contractile force of preparations stimulated at 4 Hz. The inhibitory effect of phenylephrine (1-10 microM) was partially prevented by either 1 microM yohimbine or 0.5 microM prazosin. However, it was antagonized, to about the same degree as that observed with clonidine, by the pretreatment of the preparations with both 1 microM yohimbine and 0.5 microM prazosin. The results seem to indicate that one component of the prejunctional effects of phenylephrine may be mediated by presynaptic alpha-adrenoceptors belonging to the alpha 1-subtype.