Low salt mimics effects of dark pulses on circadian pacemaker in cultured chick pineal cells

Chick pineal cells in static culture display a persistent photosensitive circadian rhythm of melatonin production and release. Pulses of white light or darkness, in otherwise constant red light, induce phase shifts in subsequent cycles whose magnitude and direction depend on the phase at which the pulse is given. Such "phase-dependent phase shifts" are mediated by effects on the underlying pacemaker. We reported previously that inhibiting the Na-K-ATPase with ouabain or salt solutions lacking potassium evokes phase shifts with the same phase dependence as those induced by pulses of darkness. One of the consequences of inhibiting the sodium pump is cell swelling. To test the relevance of this effect, we exposed chick pineal cells to pulses of medium containing reduced concentrations of NaCl, which should cause cell swelling. These hypotonic solutions induced phase shifts in the melatonin rhythm with the same phase dependence as those caused by pulses of ouabain or darkness. The size of the phase shifts varied with degree of dilution, and phase shifting was prevented by replacement of NaCl. In view of previous results showing that hypertonic media mimicked the phase-shifting effects of light, these results suggest that cell swelling may mediate the darklike effects of ouabain on the circadian pacemaker in chick pineal cells.