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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 tvas 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 mug/mg protein vs control: 0.40 +/- 0.06 mug/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 mug/mg protein vs control: 2.86 +/- 0.37 mug/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. (C) 2000 Academic Press.