Kupffer cells contain voltage-dependent calcium channels

Abstract

Kupffer cells, the resident hepatic macrophages, are activated by calcium, but conclusive evidence that they contain voltage-dependent calcium channels has not been presented previously. In this study, the cytosolic free calcium concentration ([Ca2+]i) of cultured Kupffer cells was measured with the fluorescent Ca2+ indicator fura-2. Partial replacement of extracellular Na+ by K+ caused an increase in [Ca2+]i in a concentration-dependent manner (half-maximal effect at 81 mM K+), presumably due to membrane depolarization. At 65 mM K+, where there were minimal changes in [Ca2+]i, addition of the dihydropyridine-type calcium channel agonist BAY K 8644 (1 microM) caused a large increase in [Ca2+]i. Overall, the effect of BAY K 8644 (1 microM) was to shift the concentration-response curve for K+ to the left (half-maximal effect at 61 mM K+). Under depolarizing conditions (65 mM K+), BAY K 8644 increased [Ca2+]i in a concentration-dependent manner (half-maximal effect at approximately 400 nM BAY K 8644). Moreover, the dihydropyridine-type calcium channel blocker nitrendipine inhibited the BAY K 8644-induced increase in [Ca2+]i in a concentration-dependent manner (half-maximal inhibition with about 25 nM nitrendipine). When extracellular Ca2+ was omitted from the incubation medium, the increases in [Ca2+]i due to BAY K 8644 were prevented completely. In addition, an intracellular Ca2+ antagonist, 8-(N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate hydrochloride (200 microM), did not inhibit the BAY K 8644-sensitive, voltage-dependent increase in [Ca2+]i. Thus, these data collectively indicate that BAY K 8644 causes a transmembrane Ca2+ influx in Kupffer cells in a voltage-dependent manner, providing the first direct evidence that Kupffer cells contain L-type voltage-dependent Ca2+ channels.