ENHANCED CALCIUM INFLUX BY PARATHYROID-HORMONE IN IDENTIFIED HELISOMA-TRIVOLVIS SNAIL NEURONS

The modulation of [Ca2+](i) by parathyroid hormone (PTH) has been exte nsively studied in vertebrates. The present study examined the effects of PTH on [Ca2+](i) in isolated invertebrate neurons B5 from buccal g anglia of the pond snail, Helisoma trivolvis, utilizing the Fura-2 flu orescence technique. Bovine PTH, bPTH-(1-84), induced a slow and susta ined increase in [Ca2+](i) in neurons B5. In contrast, the elevation o f extracellular K+ concentration from 1.7 mM to 15 mM induced a rapid and transient increase in [Ca2+](i). Simultaneous application of 15 mM KCl and bPTH-(1-84), or application of 15 mM KCl in the presence of b PTH-(1-84) additively increased [Ca2+](i) in neurons B5. An increase i n [Ca2+](i) in neurons B5 was also induced by a PTH agonist [bPTH-(1-3 4)], but not by a PTH antagonist [bPTH-(3-34)]. The absence of calcium , or the presence of lanthanum (2 mM) or omega-conotoxin (10 mu M), in the bath solution abolished the effect of bPTH-(1-84) on [Ca2+](i). F urthermore, our results demonstrated that the effect of PTH on [Ca2+]( i) in neurons B5 was not due to the hormonal modulation of voltage-dep endent Na+ or K+ channels or a Na+/Ca2+ exchanger. The results from th is study show that PTH can modulate [Ca2+](i) in an identified inverte brate neuron mainly by promoting extracellular calcium influx via the N-like voltage-dependent calcium channel.