Caffeine mediates cation influx and intracellular Ca2+ release in leech P neurones

We investigated the effect of caffeine on the intracellular free Ca2+ concentration ([Ca2+]i) of leech P neurones by using the fluorescent indicator Fura-2. Caffeine induced a [Ca2+]i increase that was strongly reduced, but not abolished, in Ca(2+)-free solution. The effect of caffeine on [Ca2+]i was dose-dependent: while 5 mM caffeine evoked a persistent [Ca2+]i increase that could be elicited repetitively, 10 mM caffeine or more induced a transient [Ca2+]i increase that was strongly reduced upon subsequent applications at the same concentration. Surprisingly, the cells remained fully responsive to a moderately increased caffeine concentration. The caffeine-induced [Ca2+]i increase was not blocked by millimolar concentrations of La3+, Mg2+, Cd2+, Zn2+, Co2+, Ni2+, or Mn2+. While La3+ and Mg2+ had no effect on the caffeine response, the other cations caused irreversible changes in the Fura-2 fluorescence. The inhibitors of intracellular Ca2+ pumps-thapsigargin, cyclopiazonic acid (CPA), and 2,5-di-(t-butyl)-1,4-hydroquinone (BHQ)--had no effect on the caffeine-induced [Ca2+]i increase at normal extracellular Ca2+ concentration, but they reduced it in Ca(2+)-free solution. Ryanodine had no effect on the caffeine-induced [Ca2+]i increase at normal extracellular Ca2+ concentration, and also in Ca(2+)-free solution it seemed to be largely ineffective. Caffeine evoked complete fluctuations of the membrane potential. The effect in Ca2+ free and in Na(+)-free solution suggests that the depolarizing response components were mainly due to Na+ influx, while Ca2+ reduced the Na+ influx and/or activated mechanisms which re- or hyperpolarize the cells. It is concluded that leech P neurones possess caffeine-sensitive intracellular Ca2+ stores, as well as caffeine-sensitive ion channels, in the plasma membrane that are activated by a voltage-independent mechanism. The plasma membrane channels are permeable to various divalent cations including Ca2+, and possibly also to Na+.