The present experiment was carried out to investigate the effects of exogenous adenosine 5′-triphosphate (ATP) and growth hormone (GH) on cellular H+ efflux rate (extracellular acidification rate) and Ca2+ concentration ([Ca2+]c) in cloned bovine mammary epithelial cells (bMEC) raised from the mammary gland of a 26-day-pregnant Holstein heifer. Perifusion of 2-day cultured cells with a medium containing ATP (10, 100 and 1000 μmol/l) for 30 min caused a significant and concentration-dependent increase in the cellular H+ efflux rate. ATP application (100 μmol/l) caused a transient and large increase in [Ca2+]c in all cells. In contrast, perifusion with a medium containing bovine GH at 10, 50 and 250 ng/ml for 30 min caused a significant decrease in the cellular H+ efflux rate in a concentration-dependent manner. However, bovine GH application (50 ng/ml) caused a small decrease followed by an increase, in some cases, in [Ca2+]c. In bMEC treated with lactogenic hormones (1 μg/l prolactin, 1 nmol/ml dexamethasone and 5 μg/ml insulin) for 2 days, the increased H+ efflux rate induced by ATP was significantly reduced, whereas the negative response induced by GH was inversely and significantly changed to the positive. Treatment of the cells with lactogenic hormones reduced the increase in [Ca2+]c induced by ATP stimulation, while it enhanced the increase in [Ca2+]c induced by GH stimulation. Application of ATP or GH did not cause any significant changes in [pH]c. Treatment with lactogenic hormones enhanced GH receptor (GHR) transcription that was determined by RT-PCR. From these results, we conclude that exogenous application of ATP and GH causes prompt and significant responses in H+ transport and [Ca2+]c that were significantly changed in the opposite direction by the treatment with lactogenic hormones. The lactogenic hormone treatment also enhanced GHR transcription, which may change post-receptor signal transduction systems for both agents in the bMEC.
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