Inhibition of nitric oxide facilitates LH release from rat pituitaries

Abstract

We examined the effects of nitric oxide (NO) modulators on rat pituitary LH content in vivo and studied their response to LHRH-stimulated LH secretion in vitro in ovariectomized adult female Sprague Dawley rats. Alzet mini pumps (flow rate 10 μl/h) delivering either normal saline (Group I), 1.2 mg nitroglycerin, a donor of NO (Group II) or 50 mg of nitro-L-Arginine methyl ester, a NO synthase (NOS) inhibitor (Group III), were subcutaneously implanted into experimental animals. Following 36 h infusion, pituitaries were removed and either frozen for LH quantitation, or fragmented and challenged in the superfusion system with 10 min pulses of LHRH (1 ng/ml) at 90 min intervals for 10 hours. LH was assayed by radio-immunoassay (RIA) in the homogenates of pituitaries and in aliquots of the superfusate collected every 10 mins. Significantly lower pituitary LH levels were noted in Group III (150.3 ± 18.6 ng) in comparison to Groups I (215.6 ± 5.5 ng; p < 0.04) or II (221.2 ± 14.9 ng; p < 0.01), suggesting that low levels of NO stimulate LH secretion in vivo. The pituitary LH contents were not significantly different in Groups I and II. In vitro studies reveal that exogenous LHRH stimulated response, measured as average pulse response (90 minute period after LHRH), and total LH released during the 10 hour perfusion, was 290 ± 23.6 ng and 1646.7 ± 270.8 ng, respectively, in Group III; 57.9 ± 3.1, and 344.7 ± 24.3 ng in Group I, and 105.3 ± 6.3, and 633.7 ± 77.1 mg in Group II. Thus, our in vitro studies demonstrate significantly enhanced (p < 0.05) LHRH- stimulated LH secretion in Group III in comparison to Groups I and II, while Group II shows higher responsiveness than Group I (p < 0.05). The results of the current studies provide evidence that NOS inhibition facilitates pituitary LH secretion. The differential responses to LHRH-stimulated LH secretion in vitro in the 3 groups suggest a possible role of NO in modulating pituitary LHRH receptor concentrations. However, this will have to be tested by further studies.