| Budget Amount *help |
¥2,000,000 (Direct Cost: ¥2,000,000)
Fiscal Year 1992: ¥700,000 (Direct Cost: ¥700,000)
Fiscal Year 1991: ¥1,300,000 (Direct Cost: ¥1,300,000)
|
|---|
| Research Abstract |
Somatostatin (SRIF) is a neurohypohysial peptide which inhibits growth hormone (GH) secretion. There are at least two modes of SRIF action. One is the inhibition of intracellular cAMP production and the other, the reduction of intracellular Ca^<2+> concentration ([Ca^<2+>]_i). These two phenomena occur independently. In human pituitary GH-producing cells, SRIF hyperpolarizes the membrane and thereby inhibits Ca^<2+>-dependent spontaneous action potentials. This membrane hyperpolarization is caused by G- protein coupled K^+ conductance increase. A similar membrane hyperpolarization or K^+ conductance increase has been reported in primary cultured rat somatotrophs, and clonal rat GH cells. The reduction of [Ca^<2+>]_i caused by SRIF appears to be ascribed to the inhibition of Ca^<2+> influx through voltage-gated channels during membrane hyperpolarization. However, the direct evidence for this mechanism has not yet been shown. In the present experiment we recorded the membrane potential c
… More
hange caused by SRIF in human GH-producing pituitary tumor cells using the perforated whole cell clamp technique which could prevent the wash-out of intracellular substrates. [Ca^<2+>]_i was simultaneously measured using Fura-2 AM. It has also been reported that SRIF reduces voltage-gated Ca^<2+> channel current. However under the conventional whole cell technique, wash-out may modify the effects of SRIF. In the present experiment, therefore, we also examined the effect of SRIF on voltage-gated Ca^<2+> channel current using the perforated whole cell clamp technique. An application of 10^<-8>M SRIF hyperpolarized the membrane and arrested Ca^<2+>-dependent spontaneous action potentials. [Ca^<2+>]_i concurrently decreased during membrane hyperpolarization. When the membrane potential was clamped below the threshold for voltage-gated Ca^<2+> channels, [Ca^<2+>]_i decreased and SRIF did not further reduce [Ca^<2+>]_i. In cells which did not show spontaneous action potentials, SRIF hyperpolarized the membrane but it little affected [Ca^<2+>]_i. From these results it was concluded that reduction of [Ca^<2+>]_i caused by SRIF was ascribed to the decrease of Ca^<2+> influx through voltage-gated channels during membrane hyperpolarization. The effect of SRIF on voltage- gated Ca^<2+> channel current was also examined under the perforated whole cell clamp. SRIF (10^<-8> M) inhibited Ca^<2+> channel current to 80.8-15.4% (n=5) of the control. Because SRIF-induced inhibition of voltage-gated Ca^<2+> channel current was not prominent, it was considered that membrane hyperpolarization is the major cause of the reduction of [Ca^<2+>]_i in human GH-producing cells. Less
|
