Calcium channels mediate phase shifts of theBulla circadian pacemaker

Summary

• 1. Light-induced phase advances of the activity rhythm of theBulla ocular circadian pacemaker are blocked when the extracellular calcium concentration is reduced with EGTA to 0.13 μM. Phase advances are also blocked in low calcium solutions without EGTA ([Ca]<50 μM).

• 2. The dependence of light-induced phase delays on extracellular calcium concentration in EGTA-free seawater was determined. Phase delays are blocked at calcium concentrations below 400μ M, and reduced at concentrations of 1 mM and 3.5 mM (relative to shifts in normal ASW, [Ca] = 10 mM). Phase delays are also reduced and blocked at calcium concentrations higher than normal (60 mM and 110 mM, respectively).

• 3. Low calcium EGTA also blocked both phase delays and phase advances induced by pulses of depolarizing high K⁺ seawater. Low calcium EGTA pulses presented alone at the same times did not generate significant phase shifts.

• 4. The organic calcium channel antagonists verapamil, diltiazem and nitrendipine as well as the inorganic calcium channel antagonists La³⁺, Co²⁺, Cd²⁺, and Mn²⁺ were applied along with light pulses, however, the treated eyes were either phase shifted by these substances, or these substances were found to be toxic.

• 5. The inorganic calcium channel antagonist Ni²⁺ blocked both light-induced phase delays and advances at a concentration of 5 mM. Ni²⁺ applied alone did not generate significant phase shifts. Phase delays induced by high K⁺ seawater were blocked in the presence of 50 mM Ni²⁺ but not in 5 mM Ni²⁺. The light-induced CAP activity of the putative pacemaker cells was not inhibited by Ni²⁺, suggesting that its blocking action was probably via its known role as a calcium channel antagonist.