Endothelin-1 Mediated Ca 2+ Influx Does Not Occur Through L-Type Voltage-Dependent Ca 2+ Channels in Cultured Bovine Trabecular Meshwork Cells

ABSTRACT

We evaluated whether endothelin-1-induced changes in cytosolic Ca²⁺ concentration ([Ca²⁺]_i) involve Ca²⁺ influx through L-type membrane voltage-dependent Ca²⁺ channels in cultured bovine trabecular meshwork (TM) cells. The cells were loaded with the fluorescent Ca²⁺ indicator fura-2 and cell-associated fluorescence was measured with a digital video-imaging analyzer. Application of carbachol (10⁻³ M) and norepinephrine (10⁻⁵ M) increased [Ca²⁺]_i only transiently. Endothelin-1 (10⁻⁹ M to 10⁻⁷ M) also increased [Ca²⁺]_i in a concentration-dependent manner, and its effects were larger than those of carbachol and norepinephrine. Unlike carbachol or norepinephrine, endothelin-1 evoked a peak transient effect followed by a sustained elevated level of [Ca²⁺]_i. The only level of the sustained elevated component was dependent on extracellular Ca²⁺. However, pretreatment with diltiazem (10⁻⁵ M and 10⁻⁴ M), an L-type Ca²⁺ channel blocker, did not affect either component of the endothelin-1-induced increase in [Ca²⁺]_i. These results suggest that in cultured bovine TM cells endothelin-1 receptors are coupled to Ca²⁺ signaling mechanisms. However, the extracellular Ca²⁺-dependent increase in [Ca²⁺]_i may not involve Ca²⁺ influx through L-type Ca²⁺ channels.