Electrophysiological responses to somatostatin of rat hypophysial cells in somatotroph‐enriched primary cultures.

Abstract

1. Somatotroph cells were obtained from pituitaries of adult male rats by dissociation, separation and enrichment on a continuous gradient of bovine serum albumin at unit gravity. They were kept in culture for 7‐15 days before electrophysiological experiments. 2. Immunofluorescent staining of the resulting gradient fractions (numbered F2 to F9) indicated that the majority of somatotrophs (75‐85%) were located in the heavy fractions (F8 and F9). However, a small percentage (15‐20%) of cells in these fractions were identified as lactotrophs. 3. Perifusion experiments indicated that on the one hand release of growth hormone from somatotroph‐enriched fractions was stable at the level of 6 ng (2 min)‐1 (10(6) cells)‐1 and was markedly inhibited by somatostatin (1.9 ng (2 min)‐1 (10(6) cells)‐1) but not by dopamine. On the other hand, in the same cell preparations, basal prolactin release (1.6 ng (2 min)‐1 (10(6) cells)‐1) was significantly reduced by dopamine (0.08 ng (2 min)‐1 (10(6) cells)‐1) but remained unchanged by somatostatin treatment. 4. The inhibitory effect of somatostatin on growth hormone release was dose dependent. This effect was not abolished by tetraethylammonium (40 mM) or 4‐aminopyridine (5 mM), but somatostatin decreased high‐potassium‐induced release. 5. In all the cells recorded (n = 187), 14% (n = 26) displayed a low resting potential (less than ‐30 mV) and poor membrane resistance (less than 50 M omega). The recording was unstable and resting potentials decreased regularly to 0 mV in less than 5 min. The other 86% of the cells displayed resting potentials varying from ‐45 to ‐65 mV and had a membrane resistance of more than 150 M omega. Only cells which displayed these membrane characteristics showed clear responses to somatostatin or dopamine, and were therefore chosen for experiments. 6. In all the cells selected for the experiments (n = 161), 78% (n = 126) showed either triggered or spontaneous action potentials. The action potentials remained insensitive to sodium‐free bath solution, but were reversibly blocked by the calcium channel blockers cobalt (5 mM) or nickel (5 mM). 7. When the cells were at resting potential, somatostatin induced a hyperpolarizing response associated with a decrease of membrane resistance. During this response, spontaneous or triggered action potentials were inhibited. The hyperpolarizing response induced by somatostatin was dose‐dependent.(ABSTRACT TRUNCATED AT 400 WORDS) © 1989 The Physiological Society