Fast activation and inactivation of inositol trisphosphate-evoked Ca2+ release in rat cerebellar Purkinje neurones.

1. Calcium release from stores via inositol trisphosphate (InsP3) activation of intracellular Ca2+ receptor-channels is thought to have a role in regulating the excitability of cerebellar Purkinje neurones. The kinetic characteristics of InsP3 receptor activation in Purkinje neurones are reported here. 2. InsP3 was applied by flash photolysis of caged InsP3 during whole-cell patch clamp. Ca2+ flux into the cytosol was measured with a low-affinity fluorescent Ca2+ indicator and by activation of Ca(2+)-dependent membrane conductance. 3. InsP3 produced Ca2+ release from stores with an initial well-defined delay (mean, 85 ms at 10 microM InsP3), which decreased to less than 20 ms at high InsP3 concentrations. 4. The rate of rise of free [Ca2+], which provides a measure of Ca2+ efflux and InsP3 receptor activation, increased with increasing InsP3 concentration in each cell and had a high absolute value of up to 1400 microM s-1 at 40 microM InsP3. The period of fast efflux was brief, inactivating in 25 ms at low and in 9 ms at high InsP3 concentration. 5. Peak free [Ca2+] was high (mean, 23 microM with a pulse of 40 microM InsP3) and increased with InsP3 concentration up to 80 microM InsP3 tested here. 6. Experiments with a flash-released, stable 5-thio-InsP3 confirm that the low InsP3 sensitivity of Purkinje neurones does not result from metabolism of InsP3. 7. The low functional affinity and fast activation by InsP3 suggest a difference in InsP3 receptor properties from non-neuronal cells tested in the same way. The large Ca2+ efflux and high peak [Ca2] probably result from high InsP3 receptor-channel density. 8. Elevated cytosolic [Ca2+] produced by Ca2+ influx through plasmalemmal Ca2+ channels strongly suppressed InsP3-evoked Ca2+ release from stores. Rapid termination of InsP3-evoked efflux results mainly from inhibition by high [Ca2+]. 9. The fast InsP3 activation kinetics and rapid, strong inactivation by Ca2+ influx suggest that interactions between InsP3-mediated and membrane Ca2+ signalling could occur on a time scale compatible with neuronal excitation.