Extraadrenal adrenaline formation by two separate enzymes

Angiotensin converting enzyme inhibition improves cardiac neuronal uptake of noradrenaline in spontaneously hypertensive rats

OBJECTIVES

It has been shown that a diminished sympathetic activity contributes to the hypotensive and cardioprotective actions of angiotensin converting enzyme (ACE) inhibitors (ACEI). Besides an inhibition of central sympathetic tone and peripheral noradrenaline release, we hypothesized that the interactions of ACEI with the sympathetic system may include a modulation of neuronal catecholamine uptake by peripheral nerves.

DESIGN

We investigated the influence of fosinopril on noradrenergic uptake into cardiac neurones in vitro and in vivo in acute and chronic models.

METHODS AND RESULTS

Acute administration of fosinoprilat to isolated perfused rat hearts increased the extraction of [3H]-noradrenaline from the perfusate by 39%. Treatment (14 days) of spontaneously hypertensive rats (SHR) with fosinopril (20 mg/kg per day) enhanced the cardiac uptake of i.v. administered [3H]-noradrenaline by 28%. The endogenous left ventricular content of noradrenaline was increased by 49% after an antihypertensive treatment of SHR with fosinopril (20 mg/kg per day). Identical increases in cardiac noradrenaline stores (53%) were observed in SHR treated with a blood pressure ineffective dose of fosinopril (0.2 mg/kg per day). The myocardial content of adrenaline was increased in parallel to noradrenaline after both dose regimes.

CONCLUSIONS

It is concluded that ACEI increases neuronal uptake of catecholamines in SHR in a blood pressure-independent manner. This effect occurs acutely and is independent of central sympathetic activity. Therefore, we hypothesize that ACEI modulate the activity of the cardiac noradrenaline transporter by direct activation. The improved uptake of noradrenaline may contribute to the antihypertensive and cardioprotective effects of ACEI.

Sources of human urinary epinephrine

The kidney is a likely source for some urinary epinephrine (E) since adrenalectomized animals and humans continue to excrete urinary E and the human kidney contains E synthesizing enzymes. We studied subjects during an intravenous infusion of 3H-E to determine the fraction of urinary E derived from the kidney. Eight normal subjects (CON) and 5 older, heavier hypertensives (OHH) ate a light breakfast along with ascorbic acid supplementation and had intravenous and arterial lines placed. They received an infusion of 3H-E and had an oral water load. During the final hour of 3H-E infusion, urine and arterial blood samples were collected for 3H-E and E levels. After the 3H-E infusion was abruptly discontinued, arterial blood samples were collected to measure 3H-E kinetics. The total body clearance of 3H-E was about 2,500 ml/min from plasma and clearance of 3H-E to urine was about 170 ml/min. CON had plasma E levels of 43 +/- 4 pg/ml. Their predicted rate of clearance of E from plasma to urine of 7,471 +/- 865 pg/min was less than (P = 0.018) the actual urinary E excretion of 15,037 +/- 2,625 pg/min. Thus, 43 +/- 9% of urinary E in CON was apparently derived from renal sources and not filtered from blood. Among OHH 85 +/- 4% of urinary E was derived from the kidney, significantly (P < 0.01) different from CON. The OHH also produced much more urinary E than predicted from plasma 3H-E clearance into urine (P = 0.03). A major fraction of urinary E is not filtered from the blood stream but is apparently derived from kidney. A small fraction of urinary E may be derived from E stored in nerve endings along with norepinephrine, but this probably represents less than 2% of urinary E. Renal cleavage of E sulfate into E may be another potential source of urinary E. Some, and perhaps most, urinary E not filtered from the bloodstream is derived from renal N-methylation of norepinephrine as the human kidney has two enzymes capable of converting norepinephrine to E. In conclusion, a major portion of urinary E is derived from the kidney and not filtered from the bloodstream. This is an important factor in the interpretation of urine E levels. Renal E could alter renal blood flow, electrolyte reabsorption, and renin release prior to excretion into urine.

Nonadrenal epinephrine-forming enzymes in humans. Characteristics, distribution, regulation, and relationship to epinephrine levels

Animal studies indicate that nonadrenal tissues may synthesize epinephrine (E). Here we demonstrate phenylethanolamine N-methyltransferase (PNMT) and/or nonspecific N-methyltransferase (NMT) enzymatic activity in human lung, kidney, heart, liver, spleen, and pancreas. There was a significant overall correlation (r = 0.34) between tissue PNMT and E. PNMT and NMT in human tissues differed in substrate and inhibitor specificity, thermal stability, and antigenicity. By these criteria, PNMT in human lung and in human bronchial epithelial cells were indistinguishable from adrenal PNMT. PNMT and/or NMT activity were present in red blood cells (RBCs), and cancer cell lines. Human kidney, lung, and pancreas showed immunohistochemical staining with an antibody to adrenal PNMT. RBC PNMT activity was lower in males than females and was increased in hyperthyroidism and decreased in hypothyroidism. PNMT activity in a human bronchial epithelial cell line was dramatically increased by incubation with dexamethasone. E and 3H-E levels in plasma and urine during an intravenous infusion of 3H-E into humans indicated that kidney may synthesize half of urinary E. We conclude that PNMT and NMT are widely distributed in human tissues, that they may synthesize E in vivo and are influenced by glucocorticoid and thyroid hormones.

Glucocorticoid induction of epinephrine synthesizing enzyme in rat skeletal muscle and insulin resistance

Rat skeletal muscle contains two enzymes which can make epinephrine: phenylethanolamine N-methyltransferase (PNMT) and nonspecific N-methyltransferase. We studied the time-course and mechanism by which the glucocorticoid dexamethasone increases muscle PNMT activity. We also examined the hypothesis that increased muscle E synthesis may contribute to glucocorticoid-induced insulin resistance. Dexamethasone (1 mg/kg s.c. for 12 d) increased muscle PNMT activity seven-fold but did not change NMT activity. Immunotitration with an anti-PNMT antibody indicated that the PNMT elevation was due to increased numbers of PNMT molecules. Dexamethasone rapidly increased PNMT activity and this elevation was largely maintained 6 d after glucocorticoid treatment stopped. Muscle epinephrine levels were transiently elevated by dexamethasone. Dexamethasone-treated rats had elevated insulin levels after a glucose load, and chronic administration of the PNMT inhibitor SKF 64139 reversed this increase. Chronic SKF 64139 improved glucose tolerance in normal rats. Dexamethasone induced muscle synthesis of the epinephrine-forming enzyme PNMT. A PNMT inhibitor lowered insulin levels in glucocorticoid-treated rats and glucose levels in untreated rats. These findings are compatible with antagonism of insulin-mediated glucose uptake by epinephrine synthesized in skeletal muscle.

Soluble and particulate phenylethanolamine N-methyltransferase in hypothalamus of diabetic rats

Experimental diabetes increases total phenylethanolamine N-methyltransferase (PNMT) activity in the medulla-pons but not in the hypothalamus. In this study diabetes was induced with streptozotocin (65 mg/kg) in male Sprague-Dawley rats. Twenty-eight days after treatment there were no differences in soluble PNMT activity in the hypothalamus of diabetics and controls, but PNMT activity in a membrane-associated (particulate) fraction of hypothalamus was evaluated approximately twofold in tissues of diabetic animals compared with controls. A specific PNMT inhibitor, incubated with tissue extracts of control rats, abolished greater than 90% of particulate PNMT activity in the hypothalamus but reduced soluble PNMT activity in the hypothalamus by only 47%. These findings indicate that membrane-associated PNMT activity in rat hypothalamus differs from soluble hypothalamic PNMT in the in vitro response to an inhibitor and the in vivo response to diabetes and suggest the importance of separating subcellular hypothalamic fractions prior to assay of PNMT.