Calcium oscillations in a subpopulation of S-antigen-immunoreactive pinealocytes of the rainbow trout ( Oncorhynchus mykiss)

Abstract

By means of the fura-2 technique and image analysis the intracellular concentration of free calcium ions [Ca 2+] i was examined in isolated rainbow trout pinealocytes identified by S-antigen immunocytochemistry. Approximately 30% of the pinealocytes exhibited spontaneous [Ca 2+] i oscillations whose frequency differed from cell to cell. Neither illumination with bright light nor dark adaptation of the cells had an apparent effect on the oscillations. Removal of extracellular Ca 2+ or application of 10 μM nifedipine caused a reversible breakdown of the [Ca 2+] i oscillations. Application of 60 mM KCl elevated [Ca 2+] i in 90% of the oscillating and 50% of the non-oscillating pinealocytes. The effect of KCl was blocked by 50 μM nifedipine. These results suggest that voltage-gated L-type calcium channels play a major role in the regulation of [Ca 2+] i in trout pinealocytes. Experiments with thapsigargin (2 μM) revealed the presence of intracellular calcium stores in 80% of the trout pinealocytes, but their role for regulation of [Ca 2+] i remains elusive. Treatment with norepinephrine (100 pM–50 μM), previously shown to induce calcium release from intracellular calcium stores in rat pinealocytes, had no apparent effect on [Ca 2+] i in any trout pinealocyte. This finding conforms to the concept that noradrenergic mechanisms are not involved in signal transduction in the directly light-sensitive pineal organ of anamniotic vertebrates.