EFFECT OF TMB-8 ON THE INCREASE OF INTRACELLULAR FREE CA²⁺ INDUCED BY NE AND BHQ IN DISSOCIATED SINGLE RAT BRAIN CELL

The inhibitory effect and mechanism of 8-(N,N′-diethylamino)octyl 3,4,5-trimethoxybenzoate hydrochloride (TMB-8) on the elevation of single intracellular free Ca²⁺ concentration (Ca²⁺i) induced by High K⁺, Norepinephrine(NE) and 2,5-Di(tert-butyl)-1,4-benzohydroquinone (BHQ) in dissociated single rat brain cells were studied. The changes of Ca²⁺i were reflected by the fluorescent indicator, Fura-2/AM, employed. In the absence of extracellular Ca²⁺, Ca-free Hank′s solution, preincubation with TMB-8(10,30 μmol·L^-1) for 20 min significantly decreased the resting Ca²⁺i from 79±13 nmol·L^-1 to 65±11 and 61±6 nmol·L^-1, respectively. Ca²⁺i were markedly increased by NE and BHQ and reduced significantly to control level by TMB-8. On the other hand, when the cells were incubated in Hank′s solution containing Ca²⁺ 1.3 mmol·L^-1, TMB-8(30,100 μmol·L^-1) suppressed the increase of Ca²⁺i induced by NE(0.0001～0.1 μmol·L^-1). TMB-8 showed no significant effect on Ca²⁺i elevation induced by KCl and BHQ in Hank′s solution containing Ca²⁺ 1.3 mmol·L^-1. These results indicate that TMB-8 reduced Ca²⁺i via increase of the sarcoplasmic reticulum (SR) sequestration, which blocked the release of intracellular store from the SR. However, the inhibitory effect of TMB-8 on Ca-influx from extracellular medium seems to be an indirect action from the saturation of SR with calcium.