Evidence that signal transduction for osmoreception is mediated by stretch-activated ion channels in tilapia

Prolactin (PRL) playsa central role in the freshwater osmoregulation of teleost fish,including the tilapia (Oreochromis mossambicus). Consistentwith this action, PRL release from the tilapia pituitary increases asextracellular osmolality is reduced both in vitro and in vivo.Dispersed tilapia PRL cells were incubated in a perfusion chamber thatallowed simultaneous measurements of cell volume and PRL release.Intracellular Ca²⁺ concentrations were measured from fura2-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 mannerwithout preventing cell swelling. Nifedipine, an L-typeCa²⁺ channel blocker, did not prevent the increase in PRLrelease during hyposmotic stimulation. A high, depolarizingconcentration of KCl induced a transient and marked increase ofintracellular Ca²⁺ and release of PRL but did not preventthe rise in intracellular Ca²⁺ and PRL release evoked byexposure to hyposmotic medium. These findings suggest that a decreasein extracellular osmolality stimulates PRL release through the openingof stretch-activated ion channels, which allow extracellularCa²⁺ to enter the cell when it swells.