Chronic Reserpine Pretreatment Research Articles

Increased potency of carbachol in isolated rat left atria after chronic reserpine pretreatment

Chronic reserpine pretreatment has been shown to produce postjunctional beta-adrenergic supersensitivity in cardiac tissue. The present investigation was undertaken to determine whether chronic reserpine administration also alters the heart's sensitivity to muscarinic cholinoceptor agonists. The direct and indirect (antiadrenergic) negative inotropic effects of muscarinic agonists were examined in isolated rat left atria at various times after initiation of reserpine pretreatment (1.0 mg.kg-1 x day-1 for 7 days). Supersensitivity to the positive inotropic effects of isoproterenol was demonstrated at 1, 2, and 3 wk after initiation of treatment. Concentration-response curves were then obtained for the muscarinic agonist carbachol and the adenosine A1 receptor agonist (-)-N6-(2-phenylisopropyl)-adenosine (PIA) in reducing the inotropic response to isoproterenol (3.16 microM). The potency of carbachol was increased by approximately 2.0-fold at 1, 2, and 3 wk. Also, sensitivity to the direct negative inotropic effects of carbachol was increased 1.7-fold at 1 wk but not at 2 and 3 wk. At 1 wk, reserpine pretreatment did not alter either the direct or the indirect negative inotropic effects of PIA. These results show that postjunctional beta-adrenergic supersensitivity produced by chronic reserpine pretreatment is accompanied by a reciprocal increase in sensitivity of isolated rat left atria to the negative inotropic effects of muscarinic agonists.

Cardiac alpha- and beta-adrenoceptor sensitivity and binding characteristics after chronic reserpine pretreatment.

Cardiac alpha- and beta-adrenoceptor sensitivities were examined after chronic pretreatment of rats with reserpine. Increases in sensitivity would indicate that the receptor is under the influence of the sympathetic innervation, removal by catecholamine depletion with reserpine of the tonic effect of neurotransmitter release would permit receptor upregulation. The positive inotropic responses of paced left atria and papillary muscles and the positive chronotropic responses of spontaneously beating right atria were recorded. A concentration-response curve to isoprenaline (beta-adrenoceptor-mediated) was followed, in the presence of beta-blockade, by one to methoxamine (alpha-adrenoceptor-mediated). Methoxamine exerted positive inotropy of left atria and papillary muscles, the maxima being 43.2 +/- 2.7 and 26.8 +/- 4.4% of the isoprenaline maxima. A small positive chronotropy (16.5 +/- 5.6% maximum) of right atria occurred. After pretreatment with reserpine (1.0 mg kg-1 i.p. daily) for 7 days, the three preparations displayed supersensitivity to isoprenaline, revealed as a significant displacement (P less than 0.05) of the concentration-response curves to the left of those for control rats. Reserpine pretreatment, however, had no effect on the sensitivity to methoxamine. The increase in beta-adrenoceptor sensitivity to isoprenaline after reserpine pretreatment was accompanied by a significant 41.3% increase (P less than 0.05) in the number of [3H]-dihydroalprenolol [( 3H]-DHA) binding sites (Bmax) in ventricular membranes, although the dissociation constant (KD) was unaffected. There were more alpha-adrenoceptor [3H]-prazosin binding sites in ventricular than atrial membranes. However, there was no difference in KD or Bmax between reserpine-pretreated and control tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of chronic reserpine pretreatment on myocardial sodium pump and inotropic sensitivity to ouabain in guinea pig heart

The effects of chronic reserpine pretreatment (0.1 mg/kg/day, 7–9 days) on myocardial sodium pump activity, the binding of 3H-ouabain to Na +, K +-ATPase, and the positive inotropic effect of ouabain were studied in guinea pig hearts. Ouabain-sensitive 86Rb uptake, an estimate of sodium pump activity, was significantly decreased (33.0%) in papillary muscles of chronic reserpine-pretreated guinea pigs as compared to the saline-treated controls. Kinetic analyses of the interaction of 3H-ouabain with Na +, K +-ATPase indicated that chronic reserpine pretreatment resulted in a significant decrease (24.3%) in the maximum 3H-ouabain binding site concentration when the results were expressed as pmoles per mg protein. The maximum 3H-ouabain binding sites or the number of Na +, K +-ATPase units, however, were not significantly different between the two groups when they were expressed as pmoles per mg DNA. The affinity or the dissociation rate constant (Kd) of 3H-ouabain binding was not altered after chronic reserpine pretreatment. In isolated, electrically-driven left atrial preparations, the basal contractile force was slightly higher in the reserpine-pretreated animals; the subsequent development of the positive inotropic effect and the concentration of ouabain needed to produce half-maximal inotropic response, however, were not different from the controls. Thus, it is concluded that chronic reserpine pretreatment is accompanied by a significant reduction in myocardial sodium pump activity; however, the number of sodium pump sites per cell was unchanged. The sensitivity of the reserpine-pretreated myocardium to the inotropic action of ouabain as well as its affinity for 3H-ouabain binding in vitro are also unchanged.

Beta-Adrenoceptor ligand binding and supersensitivity to isoprenaline of ventricular muscle after chronic reserpine pretreatment.

1. Isolated papillary muscles from guinea-pig hearts were paced at a constant frequency and isometric contractions reported. 2. Guinea-pigs were either untreated or pretreated with reserpine. Three pretreatment schedules were used; a) 0.5 mg kg-1 i.p. at 24 h, b) 5.0 mg kg-1 at 72 h and 3.0 mg kg-1 at 48 and 24 h, or c) 0.1 mg kg-1 daily for 7 days. 3. Cumulative concentration-response curves for the isoprenaline-induced increases in tension were obtained. The geometric mean EC50 values after the 3 and 7 day reserpine pretreatment schedules were significantly (P less than 0.05) less than for untreated guinea-pigs indicating a supersensitivity. 4. EC50 values for the positive inotropic responses to histamine and calcium in papillary muscles from reserpine-pretreated guinea-pigs did not differ significantly (P less than 0.05) from those from untreated animals. This suggests that the supersensitivity to isoprenaline is beta-adrenoceptor specific. 5. Membrane fractions were prepared from the ventricles of the untreated and reserpine-pretreated guinea-pigs from which papillary muscles had been removed. Binding of [3H]-dihydroalprenolol ([3H]-DHA) to beta-adrenoceptors of these membranes was determined. Equilibrium dissociation constants (KD) and total numbers of binding sites (Bmax) were determined by Scatchard analysis of the saturation curves for [3H]-DHA binding. 6. There was no increase in affinity (fall in KD value) or change in the total number of binding sites associated with reserpine-induced supersensitivity. The equilibrium inhibition constant (Ki) for the displacement of [3H]-DHA binding by isoprenaline was also identical in membranes from untreated and reserpine-pretreated animals. Thus reserpine-induced supersensitivity to isoprenaline does not appear to involve a change in affinity for the beta-adrenoceptor or in receptor numbers as determined by [3H]-DHA binding.

Reserpine-induced supersensitivity and the proliferation of cardiac β-adrenoceptors

Chronic reserpine pretreatment resulted in an increase in the basal developed force generated by papillary muscles isolated from the right ventricle of the rabbit. In addition, supersensitivity to the inotropic effects of isoproterenol and a concomitant increase in β-adrenoceptor number were demonstrated. These results indicate that reserpine-induced supersensitivity is not only associated with alterations at sites beyong the level of receptor activation but also at the level of specific postjunctional membrane receptors.

The role of calcium in supersensitivity to the inotropic effects of norepinephrine

Experiments using electrically stimulated rabbit left atria have demonstrated that supersensitivity to the inotropic effects of norepinephrine can be induced by either chronic reserpine pretreatment or hypothermia (lowering the temperature of the bathing medium). These two experimental conditions for inducing supersensitivity were not additive implying that they shared a common mechanism of action. Norepinephrine had no significant effect on the amplitude of a potentiated contraction of the rabbit atrium when the temperature was reduced from 37 to 30°C or following pretreatment with reserpine (30 or 37°C). Under these same conditions the ED 50 of norepinephrine on the normal contraction was reduced. It is concluded that both reserpine pretreatment and hypothermia induced supersensitivity to the inotropic effects of norepinephrine by enhancing the cellular store of activator calcium while not affecting the ability of norepinephrine to release activator calcium.

Effects of neurohumoral and adrenergic agents on cyclic amp levels in various areas of the rat brain in vitro

When norepinephrine (NE) at 10 −5 M was added to slices of various areas of the rat brain in vitro there was a 2–5 fold increase in the level of adenosine 3',5'-monophosphate (cyclic AMP) within 6 min in all areas except the cerebellum. The basal level of cyclic AMP in the cerebellum was higher than in other areas of the rat brain, and the response to NE was smaller. Histamine and serotonin (5-HT) at 10 −5 M had no effect on cyclic AMP in any brain area. However, 5-HT when added simultaneously with NE, prevented the rise in cyclic AMP elicited by NE in the hypothalamus, midbrain and brainstem. Pargyline elevated the basal levels of cyclic AMP in all brain areas but did not potentiate the action of NE. Chronic reserpine pretreatment in the rat caused an enhanced cyclic AMP response to NE. No effect on cyclic AMP in any brain area was observed following dopamine, amphetamine, or prostaglandins E 1 and E 2. Theophylline slightly reduced cyclic AMP levels. In slices of rat hypogthalamus NE, epinephrine, isoproterenol and α-methyl NE, significantly elevated cyclic AMP levels. Phenylephrine and several “false” transmitters were without effect. Further studies in the hypothalamus revealed that both α and β adrenergic blocking agents effectively antagonized the cyclic AMP response to NE. These results are somewhat difficult to interpret because the precise cellular location of the observed changes are unknown.

Further evidence for 'active' reflex vasodilation.

Further experimental evidence has been presented which strengthens the hypothesis of active reflex dilatation. The administration of a ganglionic blocking agent in a dose sufficient to block completely adrenergic tone failed to lower the perfusion pressure in the isolated extremity to the same level as was reached during the nadir of reflex dilatation. Thus, it is reaffirmed that simple withdrawal of adrenergic vascular tone (passive reflex dilatation) cannot entirely account for the total magnitude of reflex dilatation. Although chronic reserpine pretreatment eliminated both reflex constriction and dilatation, acute reserpine administration reduced reflex dilatation at a time when reflex constriction and perfusion pressure were little affected. On the other hand, the administration of the substituted phenothiazine, methotrimeprazine, eliminated adrenergic tone but had little effect on reflex dilatation. The latter two types of experiment provide further important dissociations between reflex dilatation and inhibition of existing adrenergic activity. Administration of cocaine by several different routes reduced reflex dilatation, but the reduction in reflex dilatation does not appear to be due to inhibition of reuptake of adrenergic amine. Neither does blockade of active reflex dilatation by the antihistamine, tripelennamine, appear to be related to its possible ability to block reuptake of adrenergic amine. Classic autoregulatory stimuli fail to elicit a response analogous to active reflex vasodilatation, and antihistamines fail to alter autoregulatory readjustments while reducing very effectively active reflex vasodilatation.

Species differences in amphetamine toxicity: Effects of aggregation, acute and chronic reserpine pretreatment in mice and rats

Marked differences in amphetamine toxicity between rats and mice are presented. The major differences can be summarized as follows: 1. 1. The effect of aggregation on amphetamine toxicity, while prominent in mice, is not a factor in rats. 2. 2. The protective effect of a single dose of reserpine has been confirmed in grouped mice. The rat, however, shows an increased susceptibility to some doses of amphetamine under these conditions. 3. 3. A dramatic increase in the lethality of amphetamine is produced in rats chronically treated with reserpine. This effect is equivocal in mice.

Interaction between phenylalkylamine derivatives and reserpine.

The interaction between phenylalkylamines and reserpine was studied on rats and cats be examination of the pressor and basal metabolic rate enhancing action of amphetamine, the psychostimulant and basal metabolic rate enhancing actions of ortho-bromo-methyl-amphetamine (V-104), and the pressor, psychotomimetic, basal metabolic rate enhancing and EEG-actions of para-bromo-methylamphetamine (V-111) and para-chloro-methylamphetamine(S-33). In all measurements of the effects of the compounds it was observed that if 0·5 mg/kg of reserpine was administered subcutaneously after the end of the phenylalkylamine effects, all central and peripheral effects reappeared with the equal intensity to that which could be expected from a repeated dose of the particular phenylalkylamine derivative. This phenomenon is thoughy to be due to the inactive bonding of the major part of the phenylalkylamine molecules on the receptor area from where they are released by reserpine into the active state. No such phenomenon may be observed after chronic reserpine pretreatment. Thus, it is suggested that chronic reserpine treatment enhances the sensitivity of the central nervous system to phenylalkylamines.