IBMX INDUCES CALCIUM-RELEASE FROM INTRACELLULAR STORES IN RAT SENSORY NEURONS

The intracellular free calcium concentration ([Ca2+]i) was recorded from freshly isolated rat dorsal root ganglia (DRG) neurones by means of Fura-2 or Indo-1-based microfluorimetry. Extracellular application of IBMX at millimolar concentrations evoked transient elevations in [Ca2+]i. The amplitude and rate of rise of the [Ca2+]i transient increased with increasing IBMX concentration. The effects of IBMX on [Ca2+]i were not mimicked by direct manipulation of the intracellular level of cyclic nucleotides, nor incubation with dibutyryl-cAMP, cGMP or 8-bromo-cGMP; neither did treatment with forskolin induce similar [Ca2+]i transients. The IBMX-evoked [Ca2+]i elevation persisted in Ca2+-free extracellular solutions, suggesting its origination was from intracellular stores. Caffeine-induced depletion of internal Ca2+ pool prevented IBMX-evoked [Ca2+]i responses, and vice versa, IBMX treatment strongly reduced the amplitude of subsequent caffeine-induced [Ca2+]i elevation. Both ryanodine and procaine, agents known to interact with Ca2+-gated Ca2+ release channels of the intracellular endoplasmic reticulum calcium stores, effectively inhibited IBMX-induced [Ca2+]i transients. We suggest that, in addition to its known phosphodiesterase-inhibiting activity, IBMX is an effective liberator of Ca2+ from ryanodine/caffeine-sensitive internal calcium stores in neural cells.