Veratridine-induced oscillations of cytosolic calcium and membrane potential in bovine chromaffin cells.

1. Veratridine (VTD) induced large oscillations of the cytosolic Ca2+ concentration ([Ca2+]i) and the membrane potential (Vm) in otherwise silent bovine chromaffin cells loaded with fura-2. 2. Depletion of the intracellular Ca2+ stores by thapsigargin or ryanodine did not affect these oscillations. Caffeine had a complex effect, decreasing them in cells with high activity but increasing them in cells with low activity. 3. The [Ca2+]i oscillations required extracellular Ca2+ and Na+ and were blocked by Ni2+ or tetrodotoxin. They were antagonized by high external concentrations of Mg2+ and/or Ca2+. 4. The oscillations of Vm had three phases: (i) slow depolarization (20 mV in 10-40 s); (ii) further fast depolarization (30 mV in 1 s); and (iii) rapid (5 s) repolarization. [Ca2+]i decreased during (i), increased quickly during (ii) with a 1 s delay with regard to the peak depolarization, and decreased during (iii). 5. Slight depolarizations increased the frequency of the oscillations whereas large depolarizations decreased it. 6. The Ca(2+)-dependent K+ channel blocker apamin increased the duration and decreased the frequency of the oscillations. 7. We propose the following mechanism for the oscillations: (i) the membrane depolarizes slowly by a decrease of potassium conductance (gK), perhaps due to a gradual decrease of [Ca2+]i; (ii) the threshold for activation of Na+ channels (decreased by VTD) is reached, producing further depolarization and recruiting Ca2+ channels, and inactivation of both Ca2+ and VTD-poisoned Na+ channels is slow; and (iii) gK increases, aided by activation of Ca(2+)-dependent K+ channels by the increased [Ca2+]i, and the membrane repolarizes. The contribution of the Na+ channels seems essential for the generation of the oscillations. 8. Bovine chromaffin cells have the machinery required for [Ca2+]i oscillations even though the more physiological stimulus tested here (high K+, field electrical stimulation, nicotinic or muscarinic agonists) produced mainly non-oscillatory responses.