Potassium Depolarization Elevates Cytosolic Free Calcium Concentration in Rat Anterior Pituitary Cells through 1,4-Dihydropyridine-Sensitive, ω-Conotoxin-Insensitive Calcium Channels*

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Abstract
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 mM) 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 .mu.M) on [Ca2+]i. Pretreatment with BAY K 8644 (1 .mu.M) enhanced the response in [Ca2+]i induced by K+ (25 mM). The addition of Mg2+ (30 mM) and nifedipine (1 .mu.M) lowered the resting [Ca2+]i by about 40 and 20 nM, respectively. Mg2+, nifedipine, nimodipine, Go 5438, verapamil, and ditiazem inhibited the K+ (25 mM)-induced increase in [Ca2+]i; the order of potency (and half-maximally inhibitory concentrations) were nimodipine = Go 5438 = nifedipine (.apprx. 100 nM) > verapamil (900 nM) > diltiazem (>10 .mu.M) > 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).