These experiments tested the hypothesis that inhibitory catecholaminergic neural systems suppress tonic luteinizing hormone (LH) secretion in the prepubertal ewe lamb, as they do in the adult anestrous ewe. This was done by determining the effects of the α-adrenergic antagonist, phenoxybenzamine, and the dopaminergic antagonist, pimozide. In the first experiment, pimozide at 0.24 mg/kg increased LH pulse frequency in 28-week-old lambs, but not in 17-week-old animals. Phenoxybenzamine (0.8 mg/kg) had no effect at either age. In experiment 2, a dose-response curve for pimozide (0.008, 0.08, 0.8 mg/kg) and a high dose of phenoxybenzamine were tested at two ages. The highest dose of pimozide and phenoxybenzamine stimulated LH release at both 22 and 28 weeks, although the magnitude of the response was blunted in older lambs. In the third experiment, we tested whether age or time of year determined the response to pimozide by using two groups of lambs born 5 weeks apart. Two doses of pimozide (0.24 and 0.8 mg/kg) were tested at the same time of year in these lambs. The highest dose of pimozide again stimulated LH secretion in 22-week-old lambs, but not in 17-week-old animals; the lower dose produced a partial (not statistically significant) response only in older lambs. These data suggest that inhibitory catecholaminergic (probably dopaminergic) neurons contribute to the suppression of tonic LH secretion prior to puberty in the ewe lamb, but that this neural system only becomes functional between 17 and 22 weeks of age.
Studies were performed in conscious, chronically catheterized virgin, 8- to 9-day-pregnant, and 15- to 16-day-pregnant Sprague-Dawley rats in baseline state and after removal of 7.5% total blood volume. Measurements were made of glomerular filtration rate (GFR), renal plasma flow (RPF), renal vascular resistance (RVR), arterial blood pressure (AP), and urinary electrolyte excretion. In baseline state, GFR and RPF were elevated at days 8-9 and days 15-16 of pregnancy (vs. virgins) due to a gestational renal vasodilation. The fall in hematocrit indicates substantial plasma volume expansion by days 15-16 of pregnancy. After removal of 7.5% total blood volume, little change occurred in AP in any group. However, the renal vasculature provided a sensitive response to moderate hemorrhage, since RPF fell and RVR increased similarly in virgin, 8- to 9-day- and 15- to 16-day-pregnant rats. GFR was protected in virgin and 8- to 9-day-pregnant rats but fell significantly in late pregnancy. Urinary electrolyte excretion tended to fall but was not significantly reduced by hemorrhage in any group. These studies indicate that renal vascular response to moderate hemorrhage is similar in virgin, early, and late pregnancy. Thus effector mechanisms that sense volume and regulate RVR must be continually reset to respond to progressive plasma volume expansion of pregnancy as normal.
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