MEASUREMENT OF SOMATOSTATIN RELEASE IN RAT-BRAIN BY MICRODIALYSIS

We determined the most suitable conditions for measuring the somatosta tin (SRIF) level by brain microdialysis and investigated its release f rom the hypothalamus. The relative recovery rate of SRIF was 8.4 +/- 0 .5% (mean +/- SE) using a polycarbonate (PC) membrane with the push-pu ll method at a flow rate of 2 mu l/min. Using tubes with an internal d iameter of 0.28 mm and lengths of 5, 25, 50 and 100 cm, the relative r ecovery rates using a PC membrane with the push method were 8.2 +/- 0. 5%, 7.3 +/- 0.6%, 6.2 +/- 0.5% and 4.1 +/- 0.6%, respectively. When us ing tubes with an internal diameter of 0.1 mm and lengths of 5, 25, 50 and 100 cm, the relative recovery rates were 7.3 +/- 0.7%, 5.6 +/- 1. 0%, 3.5 +/- 1.1% and 1.4 +/- 0.7%, respectively. The relative recovery rate was 5.2 +/- 0.5% with a polysulfone (PS-F, Fresenius) membrane, 4.5 +/- 0.4% with a PS-H (Hospal) membrane, 2.6 +/- 0.2% with an ethyl enevinyl alcohol membrane (EVAL), 5.1 +/- 0.8% with a polyvinyl alcoho l (PVA) membrane and 10.4 +/- 0.8% with a PS-K (Kaneka) membrane. With the push method, the extracellular SRIF level in rat pituitary was 42 .8 +/- 1.8 pg/ml with a PC membrane, 23.1 +/- 2.9 pg/ml with an EVAL m embrane at a flow rate of 2 mu l/min. With the push-pull method, it wa s 52.7 +/- 5.2 pg/ml using a PC membrane, 33.5 +/- 2.8 pg/ml using a P VA membrane and 54.4 +/- 3.2 pg/ml using a PS-K membrane. Intraperiton eal injection of urethane significantly increased SRIF from a basal le vel of 36.8 +/- 10.6 pg/ml to 66.6 +/- 12.5 pg/ml (P < 0.05) after 1 h ; however, pentobarbital decreased it from 44.7 +/- 4.1 pg/ml to 16.0 +/- 4.6 pg/ml after 1 h. These data, which give optimum microdialysis conditions for measuring SRIF release in the rat, were useful for meas uring the physiological release of SRIF from the hypothalamus.