The integration of voltage-sensitive and -insensitive calcium entry pathways in gonadotropin-releasing hormone (GnRH)-induced calcium mobilization and luteinizing hormone (LH) release were investigated in anterior pituitary gonadotrophs. Increase in calcium entry through voltage-sensitive calcium channels in nystatin-perforated cells is associated with two distinct positive effects on GnRH-evoked calcium release. It facilitates a decrease in the interspike periods that lead to increase in the frequency of spiking and also magnifies calcium signaling by increasing the duration and amplitude of calcium oscillations. The same effects are observed in cells in which calcium oscillations are induced by InsP3. Pituitary gonadotrophs also express ATP-gated receptor channels. Their activation causes calcium influx and a consequent increase in cytosolic calcium concentrations. Purinergic channels are voltage-insensitive, but their activation is associated with the depolarization of gonadotrophs and the additional facilitation of calcium entry through voltage-sensitive calcium channels. Purinergic channel-mediated calcium influx also affects agonist-induced and InsP3-dependent calcium oscillations by increasing the frequency, baseline, and duration of spiking. The effects of voltage-sensitive and ATP-gated calcium channels on the frequency of calcium spiking are consistent with the model simulation of calcium spiking, in which a small increase in cytosolic calcium concentrations is able to trigger a large pulse of calcium oscillations in the presence of a constant InsP3 concentration. Both calcium entry pathways also potentiate GnRH-induced LH release. Gonadotrophs were found to co-secrete ATP with LH, indicating the physiological significance of the expression of purinergic channels in these cells. In perifused pituitary cells, ATP is promptly degraded by ecto-ATPase, ecto-ADPase, and ecto-5' nucleotidase to adenosine in a calcium/magnesium-dependent manner. These observations indicate that ATP represents a positive feedback element in agonist-induced calcium signaling and gonadotropin secretion, and that this action is controlled by ectonucleotidases.
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