Effect of reserpine on the activity of adrenal enzymes involved in the synthesis of adrenaline

Cardiovascular vagosympathetic activity in rats with ventromedial hypothalamic obesity

OBJECTIVE

Rats with ventromedial hypothalamic lesion (VMH) are massively obese with endogenous hyperinsulinemia, insulin resistance, low sympathetic activity, and high parasympathetic activity, which are likely to induce hypertension. The goal was to follow in this model the long-term hemodynamic changes and to investigate the role of autonomic nervous system and insulin resistance in these changes.

RESEARCH METHODS AND PROCEDURES

Heart rate and blood pressure were monitored for 12 weeks after operation using a telemetric system in VMH and sham rats. Plasma catecholamines and heart beta-adrenoceptors were measured. Glucose tolerance was studied after an intravenous glucose injection and insulin sensitivity during a euglycemic hyperinsulinemic clamp test.

RESULTS

A marked bradycardia and only a mild increase in blood pressure occurred in VMH rats compared with sham animals. Response to autonomic-acting drugs showed an increase in heart vagal tone and responsiveness to a beta-agonist drug. Plasma catecholamine levels were markedly increased, and the density and affinity of heart beta-adrenoceptors were similar in VMH, sham, and control rats. Muscle glucose use was reduced by 1 week after operation in VMH animals.

DISCUSSION

These results show the following in this model of massively obese rats with sympathetic impairment: 1). adrenal medulla secretion is increased, probably as a result of hyperinsulinemia and increased vagal activity; 2). cardiac responsiveness to beta-agonist stimulation is increased; and 3). despite these changes and suspected resistance to the vasodilative effect of insulin, blood pressure does not increase. We conclude that high vagal activity may be protective against hypertension associated with obesity.

Influence of hypothyroidism on the ontogenic development of tyrosine hydroxylase induction in the adrenal glands of the rat

The ontogenic development of the transsynaptic induction of adrenal tyrosine hydroxylase (TH), evoked by reserpine and nicotine was studied in control and hypothyroid young rats, aged 3-52 days. The enzymatic induction was measured as an increase in the enzyme activity, since this increase was shown to be impaired either by an inhibitor of RNA synthesis or by a ganglionic blocker. In the control animals, TH induction elicited by reserpine increases between 3 and 32 days of age. In the hypothyroid rats, the enzymatic induction is impaired up to 32 days; at 52 days the induction is similar in both groups of animals. When nicotine is used as a stimulating agent, hypothyroidism still impairs the enzymatic induction at 5 and 21 days, indicating that at least one of the mechanisms inhibited by hypothyroidism is localized in the adrenal chromaffin cells. The present results, taken together with previous findings dealing with adrenal epinephrine secretion, show that the thyroid hormones play a crucial role in the responses of the adrenal medulla to a stimulation in the developing rat, while they have no effect in the adult.

Effect of dopamine on tyrosine hydroxylase in cultured rat adrenal medulla

Explants of rat adrenal medulla were cultured in defined medium for up to 22 hr. Addition of dopamine to the medium led to a diminution in the activity of tyrosine hydroxylase (tyrosine 3-monooxygenase; EC 1.14.16.2) in the tissue. The enzyme activity was inversely proportional to the concentration of dopamine in the culture medium. The extent of loss of tyrosine hydroxylase, as measured by immunochemical titration, corresponded to the degree of loss in enzyme activity under the same conditions. The decreased amount of enzyme protein was due to a decrease in the rate of synthesis of tyrosine hydroxylase. However, this effect was not specific in that the relative rate of tyrosine hydroxylase synthesis was not decreased. Metabolites of dopamine when added to the medium did not affect tyrosine hydroxylase activity. Two other adrenal medullary enzymes, monoamine oxidase (EC 1.4.3.4) and acid phosphatase (EC 3.1.3.2), were not affected by addition of dopamine to the medium. The results indicate that elevated cytoplasmic levels of dopamine decrease the concentration of tyrosine hydroxylase by inhibiting protein synthesis.

Effect of chronic morphine treatment on the adrenaline biosynthesis in adrenals and brain regions of the rat

Phenylethanolamine-N-methyltransferase (PNMT) activity and tissue catecholamine content were examined after 13 days morphine treatment. Prolonged morphine treatment did not alter the PNMT activity in brain regions (A1/C1 and A2/C2 cell groups, medial basal hypothalamus, median eminence). However, the enzyme activity and the adrenaline content were increased in the adrenal medulla of male rats. In parallel, morphine treatment induced adrenal hypertrophy. In female or hypophysectomized male animals the chronic morphine treatment had no effect on adrenal weight but evoked the increase of PNMT activity. It is concluded that the morphine-induced increased adrenaline biosynthesis in the adrenal gland is not fully dependent on the intact pituitary-adrenal axis and may be mediated partly by a neural mechanism (increased splanchnic nerve activity) or by a direct effect of morphine.

Tetrahydroisoquinolinecarboxylic acids and regulation of phenylethanolamine N-methyltransferase in cultured adrenal medulla

Explants of rat adrenal medullae, cultured for up to 48 hr in the presence of 3',4'-deoxynorlaudanosolinecarboxylic acid (DNLCA) or alpha-methyltyrosine, exhibited a 69% increase in phenylethanolamine N-methyltransferase (PNMT) activity as measured in dialyzed homogenates. A related tetrahydroisoquinoline, norlaudanosolinecarboxylic acid (NLCA), when added to the medium did not elevate PNMT activity. No increase in the amount of PNMT was detected by immunochemical titration of homogenates from DNLCA-treated cultured medulla, nor were there changes in rates of synthesis or degradation of the enzyme. Although DNLCA is an inhibitor of tyrosine 3-monooxygenase, it had no effect on PNMT activity when added directly to an incubation mixture in vitro. Kinetic analyses of dialyzed homogenates from explants cultured in the presence of DNLCA revealed that the Vmax of PNMT was higher than that of control tissue. There was no decrease in Km after DNLCA treatment. The increase in PNMT activity appears to be a compensatory response to depletion of medullary catecholamines by DNLCA or alpha-methyltyrosine.