Abstract
In cells dissociated from porcine anterior pituitary glands and maintained in culture for 48 h the specific secretagogue luteinizing hormone-releasing hormone (LH-RH) induces a biphasic pattern of luteinizing hormone (LH) release. A biphasic pattern of release is also induced by 57 × 10 −3 M K + and the ionophore A-23187. By reducing the availability of Ca 2+, either by omission from the medium, chelation or interfering with Ca 2+ transport across the plasma membrane, it is shown that LH release stimulated by LH-RH is much less dependent upon the availability of extracellular Ca 2+ than that stimulated by either high K + or A-23187. Nevertheless, by using a lanthanum displacement protocol to follow the influx of 45Ca 2+ it is shown that LH-RH stimulation does induce an influx of extracellular Ca 2+. Parallel experiments in which the stimulated 45Ca 2+ efflux from preloaded cells is followed confirm the influx data but suggest, in addition, that when the influx of extracellular Ca 2+ is inhibited, the peptide is able to mobilize Ca 2+ from an intracellular location. It is thus concluded that while LH release can be initiated by an increase in the intracellular level of Ca 2+, and although LH-RH stimulation does increase the permeability of the plasma membrane to Ca 2+, the stimulation of LH release by LH-RH is not dependent upon extracellular Ca 2+.
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