Am J Physiol Cell Physiol AJP: Gastrointestinal and Liver Physiology
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Am J Physiol Cell Physiol (January 22, 2003). doi:10.1152/ajpcell.00532.2002
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Submitted on November 18, 2002
Accepted on January 17, 2003

Evidence that the signal transduction for osmoreception is mediated by stretch-activated ion channels in the euryhaline tilapia, Oreochromis mossambicus

Andre P Seale1, Nurney H Richman III2, Tetsuya Hirano2, Ian M Cooke3, and E. Gordon Grau1*

1 Hawaii Institute of Marine Biology, University of Hawaii, Kaneohe, Hawaii, USA; Department of Zoology, University of Hawaii, Honolulu, Hawaii, USA
2 Hawaii Institute of Marine Biology, University of Hawaii, Kaneohe, Hawaii, USA
3 Pacific Biomedical Research Center, University of Hawaii, Honolulu, Hawaii, USA; Department of Zoology, University of Hawaii, Honolulu, Hawaii, USA

* To whom correspondence should be addressed. E-mail: grau{at}hawaii.edu.

Prolactin (PRL) plays a central role in the freshwater osmoregulation of teleost fish, including the tilapia, Oreochromis mossambicus. Consistent with this action, PRL release from the tilapia pituitary increases as extracellular osmolality is reduced both in vitro and in vivo. Dispersed tilapia PRL cells were incubated in a perifusion chamber that allowed simultaneous measurements of cell volume and PRL release. Intracellular Ca2+ concentrations were measured from fura-2- loaded PRL cells treated in a similar way. Gadolinium (Gd3+), known to block stretch-activated cation channels, inhibited hyposmotically-induced PRL release in a dose-related manner without preventing cell swelling. Nifedipine, an L-type Ca2+ channel blocker, did not prevent the increase in PRL release during hyposmotic stimulation. A high, depolarizing concentration of KCl induced a transient and marked increase of intracellular Ca2+ and release of PRL but did not prevent the rise in intracellular Ca2+ and PRL release evoked by exposure to hyposmotic medium. These findings suggest that a decrease in extracellular osmolality stimulates PRL release through the opening of stretch-activated ion channels which allow extracellular Ca2+ to enter the cell when it swells.




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