Effects of caffeine on cytoplasmic free Ca2+ concentration in pancreatic β-cells are mediated by interaction with ATP-sensitive K+ channels and L-type voltage-gated Ca2+ channels but not the ryanodine receptor

Abstract

In the pancreatic beta-cell, an increase in the cytoplasmic free Ca2+ concentration ([Ca2+]i) by caffeine is believed to indicate mobilization of Ca2+ from intracellular stores, through activation of a ryanodine receptor-like channel. It is not known whether other mechanisms, as well, underlie caffeine-induced changes in [Ca2+]i. We studied the effects of caffeine on [Ca2+]i by using dual-wavelength excitation microfluorimetry in fura-2-loaded beta-cells. In the presence of a non-stimulatory concentration of glucose, caffeine (10-50 mM) consistently increased [Ca2+]i. The effect was completely blocked by omission of extracellular Ca2+ and by blockers of the L-type voltage-gated Ca2+ channel, such as D-600 or nifedipine. Depletion of agonist-sensitive intracellular Ca2+ pools by thapsigargin did not inhibit the stimulatory effect of caffeine on [Ca2+]i. Moreover, this effect of caffeine was not due to an increase in cyclic AMP, since forskolin and 3-isobutyl-1-methylxanthine (IBMX) failed to raise [Ca2+]i in unstimulated beta-cells. In beta-cells, glucose and sulphonylureas increase [Ca2+]i by causing closure of ATP-sensitive K+ channels (KATP channels). Caffeine also caused inhibition of KATP channel activity, as measured in excised inside-out patches. Accordingly, caffeine (> 10 mM) induced insulin release from beta-cells in the presence of a non-stimulatory concentration of glucose (3 mM). Hence, membrane depolarization and opening of voltage-gated L-type Ca2+ channels were the underlying mechanisms whereby the xanthine drug increased [Ca2+]i and induced insulin release. Paradoxically, in glucose-stimulated beta-cells, caffeine (> 10 mM) lowered [Ca2+]i. This effect was due to the fact that caffeine reduced depolarization-induced whole-cell Ca2+ current through the L-type voltage-gated Ca2+ channel in a dose-dependent manner. Lower concentrations of caffeine (2.5-5.0 mM), when added after glucose-stimulated increase in [Ca2+]i, induced fast oscillations in [Ca2+]i. The latter effect was likely to be attributable to the cyclic AMP-elevating action of caffeine, leading to phosphorylation of voltage-gated Ca2+ channels. Hence, in beta-cells, caffeine-induced changes in [Ca2+]i are not due to any interaction with intracellular Ca2+ pools. In these cells, a direct interference with KATP channel- and L-type voltage-gated Ca(2+)-channel activity is the underlying mechanism by which caffeine increases or decreases [Ca2+]i.

Effects of caffeine on glucose tolerance: A placebo-controlled study

“…In fact, post-OGTT insulin levels were not different in subjects taking active treatment and in those taking placebo, and the curves of such categories of subjects were comparable even for the highest levels of circulating caffeine (Figure 2). This is partially in disagreement with the data of Lambert (Lambert et al, 1967) and of Islam (Islam et al, 1995), who found a caffeine-induced stimulation of insulin release from cultured pancreatic b-cells, and with those of Turtle, who demonstrated an increased insulin secretion with another xanthine (theophylline) (Turtle et al, 1967) and suggested that the in vivo insulin response to caffeine may be completely different from that detected in vitro.…”

Ca2+-induced Ca2+Release from Internal Stores in INS-1 Rat Insulinoma Cells

“…Another key property of RyRs is that caffeine at millimolar concentrations can pharmacologically cause channel opening. In some studies, caffeine did not activate Ca 2＋ release from the internal stores of β-cells [14,15], whereas Ca 2＋ release from the internal stores was activated if caffeine-sensitive RyRs were present in intact β-cells [16,17]. In fact, the presence of RyRs in insulin secreting β-cells is well-accepted at the present time, although the expression of each type of RyRs is variable, depending on the species studied.…”

Caffeine-Induced Ca 2+ Transients and Exocytosis in Paramecium Cells. A Correlated Ca 2+ Imaging and Quenched-Flow/Freeze-Fracture Analysis

“…Besides mobilization of intracel-lular Ca 2+ pools, a variety of other effects have been discovered with caffeine, e.g., activation (Steenbergen & Fay, 1996) or inhibition of Ca 2+ sequestration (Chapman & Tunstall, 1988;Bassani, Bassani & Beers, 1994) or Ca 2+ release (Missiaen, Taylor & Berridge, 1992) as well as some other unrelated effects (Gupta et al, 1990;Berridge, 1991;Sawynok & Yaksh, 1993;Tanaka & Tashjian, 1993;Combettes, Berthon & Claret, 1994;Islam et al, 1995). Despite these uncertainties caffeine has remained a popular tool in secretion studies, partly because it is easy to apply and partly because of the lack of physiological alternatives.…”