Ouabain (or salt solution lacking potassium) mimics the effects of dark pulses on the 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. Here, we describe the effects of ouabain, a specific inhibitor of Na,K-ATPase, and of salt solutions lacking potassium (SS-K), which also inhibit the pump, on the melatonin rhythm. Pulses of ouabain, or of SS-K, induced phase advances and phase delays that were phase and concentration-dependent. The relationship between time of treatment and effect on the subsequent phase of the rhythm (the phase-response curve) for these treatments was essentially the same as that for pulses of darkness.