Adrenomedullary secretion of DOPA, catecholamines, catechol metabolites, and neuropeptides.

Catecholamines and their metabolites have been proposed as markers of sympathetic nervous system stimulation. However, the adrenal medulla is a rich source of catecholamines and catecholamine metabolites and may play a significant role in plasma levels of these compounds. In addition to adrenal catecholamine metabolite efflux, the role of the catecholamine precursor 3,4-dihydroxyphenylalanine (DOPA) has not been fully evaluated. The simultaneous effluxes of catecholamines, metabolites, DOPA, and neuropeptides were measured in perfusates from isolated dog adrenals. The relative abundance of compounds detected consistently during unstimulated conditions was epinephrine >> norepinephrine > 3,4-dihydroxyphenylglycol > metanephrine > normetanephrine > dopamine > 3,4-dihydroxyphenylacetic acid > 3-methoxy-4-hydroxyphenylglycol > or = DOPA >> [Met]enkephalin >> neuropeptide Y. Effluxes of analytes were not affected by cocaine and the ratios of catecholamines to metabolites increased dramatically with carbachol stimulation, consistent with negligible reuptake into adrenal cells. Thus, most of the 3,4-dihydroxyphenylglycol is expected to be derived from epinephrine and norepinephrine subsequent to translocation from chromaffin vesicles into the cytosol. The efflux of DOPA increased dramatically during stimulation with 30 microM carbachol in a calcium-dependent manner. Efflux of DOPA during the initial stabilization period of the perfusion preparation declined exponentially, in parallel with the effluxes of the catecholamines and neuropeptides but not with metabolites. Evoked release of DOPA was Ca2+-dependent. These data suggest that DOPA can be stored and released exocytotically from chromaffin granules.

Autocrine/paracrine regulation of renal Na(+)-phosphate cotransport by dopamine.

We tested the hypothesis that dopamine (DA) acts as an autocrine/paracrine regulator of Na(+)-Pi symport in proximal tubules, using opossum kidney (OK) cells as an in vivo model. Both DA and parathyroid hormone (PTH) increased adenosine 3',5'-cyclic monophosphate (cAMP) and inhibited Na(+)-gradient-dependent uptake of 32P but not that of L-[3H]-alanine. Incubation of OK cells with L-dopa, a DA precursor, resulted in accumulation of DA (7.4 nM), a ninefold increase of cAMP in the medium, and an inhibition (-10%) of Na(+)-Pi uptake. Carbidopa, an inhibitor of aromatic-L-amino acid decarboxylase, prevented the formation of DA from L-dopa, the increase in cAMP, and the inhibition of Na(+)-Pi cotransport. Pi-replete OK cells produced more DA (+15%) from L-dopa than Pi-deprived cells; however, the endogenous DA inhibited Na(+)-Pi cotransport both in Pi-deprived and in Pi-replete cells. Thus OK cells can synthesize DA from L-dopa in a quantity sufficient to elicit both the maximum DA-stimulated cAMP accumulation and inhibition of Na(+)-Pi cotransport in the same cell population. Our data, obtained on an in vitro system, support the hypothesis proposing that DA generated in proximal tubular cells can modulate, via cAMP, the Na(+)-Pi symport in the same or adjacent cells. If present in the kidney, this pathway might represent an autocrine/paracrine system that can contribute to regulation of renal Pi homeostasis.

Dopamine enhances the phosphaturic response to parathyroid hormone in phosphate-deprived rats.

Phosphate deprivation results in a resistance to the phosphaturic effect of parathyroid hormone. Dopamine is phosphaturic and is synthesized by kidney proximal tubule, the nephron subsegment where parathyroid hormone inhibits phosphate transport. Thus, to test the hypothesis that phosphate deprivation is associated with low intrarenal dopamine synthesis and that dopamine infusion will overcome the resistance to the phosphaturic response to parathyroid hormone, the following study was performed. The effect of dietary phosphate intake on intrarenal dopamine synthesis, as reflected by urinary dopamine excretion, was determined. Rats were placed in metabolic cages (N = 5) and were fed a low-phosphate diet (0.07% Pi) for 4 days and then a high-phosphate diet (1.8% Pi) for 4 days. Twenty-four-hour urinary dopamine excretion was significantly lower in rats fed a low-phosphate diet (2.53 +/- 0.06 versus 4.10 +/- 0.30 micrograms/day). Further, the effect of dopamine infusion on the blunted phosphaturic response to parathyroid hormone was studied in rats fed a low-phosphate diet for 1, 2, and 3 days. Control clearances were taken 2 h after thyroparathyroidectomy; then, parathyroid hormone (33 U/kg plus 1 U/kg/min), dopamine (25 micrograms/kg/min), or parathyroid hormone plus dopamine were infused for 60 min. Changes in the fractional excretion of phosphate were significantly greater in rats fed a low-phosphate diet infused with parathyroid hormone plus dopamine than in rats fed a low-phosphate diet infused with parathyroid hormone alone (delta 27.9 +/- 5.8 versus 11.2 +/- 2.6% for day 1; 28.4 +/- 1.4 versus 7.1 +/- 3.6% for day 2; and 10.7 +/- 2.8 versus -0.2 +/- 0.2% for day 3; N = 5 for all groups).(ABSTRACT TRUNCATED AT 250 WORDS)

Precursors and metabolites of norepinephrine in sympathetic ganglia of the dog.

3,4-Dihydroxyphenylalanine, dopamine, epinephrine, 3,4-dihydroxyphenylglycol, and 3,4-dihydroxyphenylacetic acid as well as norepinephrine were measured in dog lumbar sympathetic ganglia. The responses of these compounds to several classes of stimuli were investigated using an isolated time-resolved superfusion system. Nonselective (i.e., amphetamine and high K+) and receptor-mediated selective (oxotremorine) stimuli were used to evoke releases. The overflows of all compounds were measured by HPLC with electrochemical detection. The efficiency of each stimulus was estimated by normalizing the amount of evoked release to the total neurotransmitter pool when the stimulus was applied; i.e., fractional release was calculated. Overflows of all compounds except 3,4-dihydroxyphenylalanine were enhanced by a 10-min 100 microM amphetamine stimulus, and each of the catecholamine pools (dopamine, norepinephrine, and epinephrine) was affected to the same degree. By contrast, the 3,4-dihydroxyphenylalanine and dopamine pools were more readily releasable than the norepinephrine pool with a 10-min 80 mM K+ stimulus, and these releases were Ca2+ dependent. Epinephrine was released in preference to norepinephrine by a 10-min 1 mM oxotremorine stimulus. The data suggest the existence of at least three types of neurons in dog lumbar ganglia and are consistent with previous histological observations.