Simultaneous measurement of changes in the membrane potential and the intracellular Ca2+ concentration caused by somatostatin in human GH-producing pituitary tumor cells

Abstract

Changes in the membrane potential and the intracellular Ca2+ concentration ([Ca2+](i)) caused by somatostatin (SRIF) were simultaneously measured in human GH-producing pituitary tumor cells, by means of the nystatin-perforated whole cell clamp technique and Fura-2 AM. An application of 10-8M SRIF hyperpolarized the membrane and arrested Ca2+-dependent spontaneous action potentials. [Ca2+](i) concurrently decreased during membrane hyperpolarization. When the membrane potential was clamped below the threshold for voltage-gated Ca2+ channels, [Ca2+](i) decreased and SRIF did not further reduce [Ca2+](i). In cells which did not show spontaneous action potentials, SRIF hyperpolarized the membrane but it affected [Ca2+](i) little. From these results it was concluded that the reduction in [Ca2+](i) caused by SRIF was ascribed to the decrease in Ca2+ influx through voltage-gated channels during membrane hyperpolarization. The effect of SRIF on the voltage-gated Ca2+ channel current was also examined under the perforated whole cell clamp. SRIF (10-8 M) inhibited the Ca2+ channel current to 80.8 ± 15.4% (n = 5) of the control. Because SRIF-induced inhibition of the voltage-gated Ca2+ channel current was not prominent, it was considered that membrane hyperpolarization is the major cause of the reduction in (Ca2+](i) in human GH-producing cells.