Simultaneous Measurement of Changes in the Membrane Potential and the Intracellular Ca²⁺ Concentration Caused by Somatostatin in Human GH-Producing Pituitary Tumor Cells

Abstract

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^-8M) 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.