Pfltigers Arch-Eur J Physiol (1996) 431 : 443-45t © Springer-Verlag 1996 Francisco Barros • Donato del Camino • Luis A. Pardo Pilar de la Pefia Caffeine enhancement of electrical activity through direct blockade of inward rectifying K ÷ currents in GH3 rat anterior pituitary cells Received: 31 July 1995/Received after revision and accepted: 24 August 1995 Abstract Treatment of rat anterior pituitary GH3 cells with caffeine causes a reversible enhancement of elec- trical activity superimposed over a depolarization of the plasma membrane potential. Similar results are obtained with theophylline, but not with isobutyl- methylxanthine or forskolin. The effects of caffeine are not related to Ca 2+ liberation from intracellular stores since they are not affected by incubation of the cells with ryanodine or thapsigargin. Furthermore, caffeine- induced hyperpolarization of the membrane is not detectable even in cells in which Ca 2+ liberation from inositol 1,4,5-trisphosphate-sensitive compartments produces a prominent transient hyperpolarization in response to thyrotropin-releasing hormone. Reductions of Ca2+-dependent K + currents caused by partial block of L-type Ca 2+ channels by caffeine are not sufficient to explain the effects of the xanthine, since the results obtained with caffeine are not mimicked by direct blockade of Ca 2+ channels with nisoldipine. GH3 cell inwardly rectifying K + currents are inhibited by caffeine. Studies on the voltage dependence of the caffeine-induced effects indicate a close correlation between alterations of electrical parameters and reported values of steady-state voltage dependence of inactivation of these currents. We conclude that, as pre- viously shown for thyrotropin-releasing hormone, mod- ulation of inwardly rectifying K + currents plays a major role determining the firing rate of GH3 cells and its enhancement by caffeine. Key words Caffeine • Anterior pituitary • GH3 cells- Intracellular Ca 2+ stores' Electrical activity F. Barros ((~) • D. del Camino - L. A. Pardo. P. de la Pefia Departamento de Biologia Funcional - Area de Bioquimica, Facultad de Medicina, Universidad de Oviedo, E-33006 Oviedo, Spain Introduction Elevations of intracellular Ca 2+ levels ([Ca2+]i) consti- tute an important component of the excitation-secre- tion coupling cascade in GH3 cells, leading to exocytotic release of prolactin and growth hormone [7, 13, 16, 25]. Both Ca 2+ influx through voltage- or lig- and-gated Ca 2+ channels in the plasma membrane, and Ca 2+ mobilization from intracellular storage organelles are implicated in the regulation of [Ca2+]i in response to secretagogues. Thus, thyrotropin-releasing hormone (TRH) induces in these cells an acute enhancement of hormone secretion followed by a plateau phase of sus- tained enhancement, which are primarily dependent on release of Ca 2+ from the intracellular stores and on influx ofextracellular Ca 2+, respectively [16, 23, 33, 36]. The initial phase of [Ca2+]i and secretion increases is due to production of intracellular mediators, that lib- erate Ca 2+ from intracellular compartments which are either sensitive or insensitive to inositol 1,4,5-trispho- sphate [Ins(1,4,5)P3] [23]. Spontaneous and hormone- evoked [Ca2+]i fluctuations dming the second phase of TRH action have been described in GH3 cells [4, 9, 24, 35, 36]. However, the mechanisms (or mechanism) that underlie the generation of these oscillations remain controversial. The occurrence of spontaneous or hor- mone-evoked action potentials able to cause transient increases in [Ca2+]~ has been well documented in pitu- itary cells [4, 22, 29, 36]. However, the relative con- tribution of extracellular Ca 2+ entry through voltage-dependent Ca 2+ channels and Ca 2+ release from intracellular pools to spontaneous or hormone-trig- gered [Ca2+]i oscillations in different adenohypophyseal cells has not been totally clarified [5, 10, 30, 35]. Among other drugs, caffeine has been used as a major tool to elucidate the possible relationship between intracellular Ca 2+ mobilization and a given physiolog- ical response. It is generally considered that caffeine acts by releasing Ca 2+ from an intracellular store reg- ulated by a mechanism of CaZ+-induced Ca 2+ release