INTRAVENOUS ANESTHETICS INHIBIT CEREBELLAR GUANYLATE CYCLASE ENZYME

Abstract

S427 INTRODUCTION: In the central nervous system, Nitric oxide (NO) mediates the action of glutamate acting at N-methyl-D-aspartate (NMDA) receptors. Pretreatment with nitric oxide synthase (NOS) enzyme inhibitors, Nomega nitro-L-arginine (NAG) and its methyl ester (NAME), augments intravenous anesthetics-induced loss of righting reflex duration [1] and hypothermia [2]. In NAME and NAG pretreated animals, cGMP inhibitory responses to anesthetics like ketamine and riluzole are augmented [3]. In an effort to elucidate the mechanisms of anesthetic interaction with NO-guanylate cyclase pathway, this study addressed the question, what is the direct effect of intravenous anesthetics on the guanylate cyclase?. METHODS: After institutional approval of the protocol, cerebellar guanylate cyclase enzyme was prepared [4] from adult male Swiss Webster mice (weight 20-25 g). An aliquot of the enzyme suspension ([similar]50 [micro sign]g of protein) was mixed in a reaction buffer containing known modulators of the enzyme (10 mM of S-Nitroso-N-acetylpenicillamine - SNAP, or theophylline) or intravenous anesthetic (100 [micro sign]M). The reaction was initiated by adding 1 mM GTP and incubating at 37[degree sign] C for 15 minutes. At the end of incubation, cGMP formed was extracted and assayed using radioimmunoassay RESULTS: Drugs used, significantly (ANOVA) modulated the enzyme activity in both soluble (F5,18 171, P < 0.0001) and particulate (F5,18 66, P < 0.0001) fractions. The NO generator, SNAP (a known stimulator of the enzyme), significantly augmented the cGMP production (P < 0.005) while, theophylline (a known inhibitor the enzyme) inhibited the cGMP production (P < 0.005) in both soluble and particulate fractions. Moreover, all the anesthetics (used in concentration closer to their clinical concentrations [similar] 50 - 100 [micro sign]M) inhibited the enzyme activity (P < 0.005) in both cytosolic and particulate fraction. (Figure 1)Figure 1DISCUSSION: Our results demonstrate that intravenous anesthetics may directly inhibit the enzyme guanylate cyclase in both soluble and particulate fractions. Soluble form is the only known receptor for NO. These results establish the enzyme guanylate cyclase as a target site for the intravenous anesthetic action.