Cardiac natriuretic peptide response to water restriction in the hormonal adaptation of two semidesert rodents from West Africa (Steatomys caurinus, Taterillus gracilis).

Two African rodents, Taterillus gracilis and Steatomys caurinus, native to regions of alternate dry and wet seasons, were studied under laboratory conditions. These species differ in estivation behavior, one undergoing pseudoestivation and the other strong estivation. One group of animals of each species was provided with unlimited access to seed and vegetables rich in water, mimicking the food availability of the wet season (control group). A second group of animals of each species was subjected to water restriction for 8 days, mimicking the natural drought that occurs during the dry-hot season. The effects of water restriction on osmoregulation and body water content were assessed from hematocrit, and plasma and urinary osmolalities (PO, UO). Whether the natriuretic peptide system was modified by the osmoregulator adaptation to aridity of these semidesert rodents was examined from measurements of atrial natriuretic peptide (ANP) levels in plasma, atria, and ventricles, in parallel with morphological studies. In both species, UO was increased by water restriction. In water-deprived T. gracilis, ANP levels were about twice (right atria: 1.08 +/- 0.16 microg/mg protein vs control: 0.40 +/- 0.06 microg/mg protein) and plasma concentrations half (0.28 +/- 0.06 ng/ml vs control: 0.64 +/- 0.07 ng/ml) those in control animals. In S. caurinus these variables were not affected by water availability (right atria water restricted: 2. 20 +/- 0.15 microg/mg protein vs control: 2.86 +/- 0.37 microg/mg protein; plasma ANP water restricted: 0.80 +/- 0.12 ng/ml vs control: 0.90 +/- 0.16 ng/ml). Consistent with these quantitative results, immunohistochemical and ultrastructural observations showed an increase in immunostaining for both the N- and the C-terminal ANP and a larger number of granules in the atria of T. gracilis following water restriction, whereas there was no visible change in S. caurinus. Thus, water restriction induced a decrease in ANP secretion in T. gracilis, increasing cardiac storage alongside a reduced urine production. In contrast, in S. caurinus, the natriuretic system was not affected by an 8-day period of water restriction.

Effect of a 14-day hindlimb suspension on beta-adrenoreceptors in rats.

Exposure to long-term simulated microgravity exhibits reduced sympathetic nervous system activity. This study tested the hypothesis that the hypersensitivity of adrenoreceptors would explain partly many other features of the hemodynamic consequences of return from space. The biochemical properties of the beta adrenoreceptors (betaAR) were determined using 125I-cyanopindolol (125I-CYP) binding in three rat groups: (1) The first experimental group consisted of 24 h-restrained orthostatic rats in the horizontal position, to test the early effect of the attachment to the suspension device; (2) the second experimental group consisted of 24 h-restrained antiorthostatic rats, to test the early effect of the suspension; (3) the third experimental group consisted of 14 day-restrained antiorthostatic rats, to test the long term effect of the suspension. The study was performed in two organs involved in blood pressure regulation, i.e. the heart (atria and ventricles were separated) and kidneys. The Scatchard analysis of 125I-cyanopindolol binding in both organs indicated no significant alterations in the dissociation constant (Kd) and the maximum binding capacity (Bmax) in the three experimental groups. These results do not allow the conclusion about the SNS adaptation pattern to simulated microgravity. Thus, the hypothesis that betaAR are involved in the cardiovascular adaptation to simulated microgravity is not verified in this model where, as a matter of fact, cardiovascular deconditioning is not verified even if this model is widely used.

Presence of atrial natriuretic peptide in two desert rodents: comparison with rat.

Atrial natriuretic peptide (ANP) was characterized and assayed in plasmas, hearts, and brains of two Algerian desert rodents, Psammomys obesus and Meriones libycus along with vasopressin, which was assayed in hypophyses and hypothalami. Using reverse-phase high-performance liquid chromatography and radioimmunoassay, we showed, in plasmas and hearts of both species of desert rats, the presence of peptides similar to rat N- and C-terminal ANP but in lower amounts than in Wistar rats. Conversely, C-terminal ANP was abundantly detected in hypophyses from Meriones libycus rats. As these peptides, through their diuretic and natriuretic activities, are involved in body fluid regulation and electrolyte balance, the reduction of ANP stores in both plasmas and hearts suggests that diuresis and natriuresis are lowered in both species of mammals adapted to arid environments. This could occur because of the vasopressin-mediated adaptation, but also in response to the low ANP involvement in hydro-osmotic regulations, even in Psammomys, which has a dietary salt loading. On the other hand, the higher C-terminal ANP contents in the hypophysis of Meriones than in Psammomys and Wistar rats remain to be understood.