Endocrinology, Vol 122, 2764-2770, Copyright © 1988 by Endocrine Society

ARTICLES

Potassium depolarization elevates cytosolic free calcium concentration in rat anterior pituitary cells through 1,4-dihydropyridine-sensitive, omega-conotoxin-insensitive calcium channels

K Meier, W Knepel and C Schofl
Department of Pharmacology, University of Freiburg im Breisgau, Federal Republic of Germany.

Changes in membrane potential may influence Ca2+-dependent functions through changes in cytosolic free calcium concentration [( Ca2+]i). This study characterized pharmacologically those voltage-dependent Ca2+ channels in normal rat anterior pituitary cells that are involved in the elevation of [Ca2+]i upon high potassium-induced membrane depolarization. The [Ca2+]i was monitored directly by means of the intracellularly trapped fluorescent indicator fura-2. The addition of K+ (6-100 mM) increased [Ca2+]i in a concentration-dependent manner. The fluorescent signal reached a peak within seconds and then decayed to form a new elevated plateau. K+ at the highest concentration used (100 mM) raised [Ca2+]i by about 450 nM. The K+-induced increase in [Ca2+]i was absent in a Ca2+-free medium. BAY K 8644, a 1,4- dihydropyridine Ca2+ channel agonist, also caused an increase in [Ca2+]i. The maximum response in [Ca2+]i upon stimulation with BAY K 8644 (100 nM) was about 40 nM. The half-maximally effective concentration of BAY K 8644 (100 nM) was about 20 nM. The response in [Ca2+]i upon BAY K 8644-stimulation was abolished in a Ca2+-free medium. Predepolarization with various K+ concentrations enhanced the effect of BAY K 8644 (1 microM) on [Ca2+]i. Pretreatment with BAY K 8644 (1 microM) enhanced the response in [Ca2+]i induced by K+ (25 mM). The addition of Mg2+ (30 mM) and nifedipine (1 microM) lowered the resting [Ca2+]i by about 40 and 20 nM, respectively. Mg2+, nifedipine, nimodipine, Go 5438, verapamil, and diltiazem inhibited the K+ (25 mM)- induced increase in [Ca2+]i; the order of potency (and half-maximally inhibitory concentrations) were nimodipine = Go 5438 = nifedipine (approximately 100 nM) greater than verapamil (900 nM) greater than diltiazem (greater than 10 microM) greater than Mg2+ (6 mM). Omega- Conotoxin (100 nM) did not inhibit the K+ (25 mM)-induced increase in [Ca2+]i. These data demonstrate that, over a wide range, membrane depolarization induced by high potassium concentration is indeed associated with increases in [Ca2+]i in normal rat anterior pituitary cells. This elevation of [Ca2+]i is mainly due to an influx of Ca2+ through 1,4-dihydropyridine-sensitive, omega-conotoxin-insensitive calcium channels (L-type).

This article has been cited by other articles:

				D. Yokomaku, T. Numakawa, Y. Numakawa, S. Suzuki, T. Matsumoto, N. Adachi, C. Nishio, T. Taguchi, and H. Hatanaka Estrogen Enhances Depolarization-Induced Glutamate Release through Activation of Phosphatidylinositol 3-Kinase and Mitogen-Activated Protein Kinase in Cultured Hippocampal Neurons Mol. Endocrinol., May 1, 2003; 17(5): 831 - 844. [Abstract] [Full Text] [PDF]