This review concentrates on the characteristics and functionality of endocrine neurons in the hypothalamo-neurohypophysial system, coexpressing two peptides, vasopressin and apelin. Vasopressin is synthesized in the soma of magnocellular neurons, then packaged in granules with its respective receptors. In these neurons, apelin is generated from a larger precursor proapelin and is detected in vesicles, some of them colocalize with vasopressin, for others there is a marked segregation of apelin and vasopressin immunoreactivity along the hypothalamo-hypophyseal axons. Furthermore, apelin receptors, like V1a-type and V1b-type vasopressin receptors, are synthesized by magnocellular vasopressin neurons. In lactating rodents, apelin given intracerebroventricularly inhibited the phasic electrical activity of vasopressin neurons, reduced plasma vasopressin levels and increased aqueous diuresis, showing that apelin acts as a potent diuretic neuropeptide, counteracting vasopressin actions through inhibition of vasopressin neuron activity and vasopressin release. Moreover, in response to potent physiological stimuli known to evoke increased phasic activity of vasopressin neurons (hyper-osmolarity like during dehydration), both the soma dendrites and neurohypophysial terminals loose their dense staining quality, and vasopressin is released by (i) dendrites in the extracellular space to optimize the characteristic phasic activity necessary to a sustained release of vasopressin and (ii) by terminals in blood circulation where vasopressin then ensures its main endocrine actions at kidney level (antidiuretic effect). Conversely, apelin accumulates in these neurons rather than being released into the bloodstream and probably into the nuclei. Thus, decreases in the local supply of apelin to magnocellular vasopressin cell bodies may facilitate the expression by vasopressin neurons of an optimized phasic activity, by decreasing the inhibitory actions of apelin on these neurons. Antagonistic regulation of apelin and vasopressin has a biological purpose, making it possible to maintain the water balance of the organism by preventing additional water loss via kidneys. This reveals a new physiological concept of dual and opposite functional potentiality for endocrine neurons coexpressing different neuropeptides in separate vesicles: depending on the degree of their electrical activation/inhibition, neurons release selectively the very coexpressed peptides that will ensure its accurate endocrine functions in perfect accordance with the hormonal demand.

Opposite potentiality of hypothalamic coexpressed neuropeptides, apelin and vasopressin in maintaining body-fluid homeostasis.

Comparative pharmacology of bovine, human and rat vasopressin receptor isoforms.

These studies were designed to characterize the atrial natriuretic peptide (ANF) receptor subtypes (guanylyl cyclase GC-A and GC-B and ANF-C) in normal sheep kidneys and to evaluate alterations in receptor kinetics during pregnancy. Kidneys were obtained from 12 nonpregnant and 12 pregnant sheep during late gestation and maintained on a 100 mmol/day salt intake. Competition binding receptor assays using [125I]human ANF showed that inner medullary membranes are exclusively of the GC-A subtype. The maximum binding capacity (Bmax, 109 +/- 12 vs. 89 +/- 18 fmol/mg protein) and dissociation constant (Kd, 240 +/- 70 vs. 324 +/- 99 pM) are not altered by pregnancy. Specific binding of glomerular membranes to [125I]Tyr-C-type natriuretic peptide, which shows the highest affinity toward GC-B receptors, was observed, but this binding was abolished when ANF-C receptors were saturated with excess C-ANF-(101-121), suggesting that [125I]Tyr-C-type natriuretic peptide binding was mediated by ANF-C receptors. Binding of [125I]human ANF to glomerular membranes revealed that glomerular ANF receptor number was reduced during pregnancy (1040 +/- 212 vs. 335 +/- 42 fmol/mg protein; P = 0.001), but binding affinity was not changed. The reduced number was mainly due to a decrease in ANF-C receptor density (832 +/- 213 vs. 260 +/- 31 fmol/mg protein; P = 0.005). Autoradiography of whole kidney frozen sections produced similar findings. These studies demonstrate that GC-B receptors are absent from renal glomeruli and inner medulla, and that ANF receptor subtypes are differentially regulated in the pregnant sheep kidney, suggesting a role for ANF in the altered volume and pressure homeostasis of pregnancy.

Regulation of renal atrial natriuretic peptide receptors in pregnant sheep.

Chlorpropamide alters AVP-receptor binding of rat renal tubular membranes.

Surgical critical care.

The mechanisms of action of antidiuretic hormone.

Adenylate cyclase has been found in membranes from all animal cells studied thus far. The catalytic activity of the enzyme can be modified in vitro by a large variety of chemical and physical agents. Some of these agents are active on adenylate cyclases from almost all cell types; examples of such agents are sodium fluoride (1) and to a lesser extent guanylnucleotides (2). Other agents, e. g., many peptide hormones, several neurotransmitters, and prostaglandins are active on the adenylate cyclase from one or a limited number of cell types. It is generally accepted that the specificity of the response is due to the presence on the membrane of specific receptor molecules. These receptors are able (1) to bind the regulatory agent and (2) to initiate the sequence of molecular events leading to a change (in most cases an increase) in the catalytic activity of adenylate cyclase. In the past few years considerable effort has been devoted to the characterization of membrane receptors functionally coupled to adenylate cyclase (for review, see Ref. 3). Some of these receptors have been extracted from the membrane in an active form. However, in none of the receptor adenylate cyclase systems investigated so far were the receptors present in a sufficiently great amount to allow complete purification and physicochemical characterization. On the other hand, very little is known about adenylate cyclase itself and about the molecular mechanisms involved in its activation by specific and nonspecific regulatory agents Almost all available data were derived from kinetic studies using intact membranes. Adenylate cyclases from various sources were found to be extractable from the membrane under the influence of nonionic detergents (4–6). However, it was not possible to obtain a purified and stable form of the soluble enzyme. Furthermore, the receptor-mediated enzyme activation was lost during membrane solubilization.

Vasopressin-sensitive Adenylate Cyclase From the Mammalian Kidney: Mechanisms of Activation

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The mechanisms of action of antidiuretic hormone.

Vasopressin-sensitive Adenylate Cyclase From the Mammalian Kidney: Mechanisms of Activation