An investigation of the sensitivity of the ouabain-insensitive sodium efflux in single barnacle muscle fibers to pentachlorophenol

Abstract

The aim of the present work was to explore the possibility that pentachlorophenol (PCP) influences the behavior of the resting Na efflux in single muscle fibers from the barnacle, Balanus nubilus. It is shown here that PCP causes a transitory rise in the Na efflux in both unpoisoned and ouabain-poisoned fibers and that the response is dose-dependent, the minimal effective concentration in ouabain treated fibers being less than 10 −6 m. The efficacy of PCP is significantly greater than that of 2,3,4-trichlorophenol. 2,3-Dichlorophenol is ineffective. This is also the case with phenol. The magnitude of the response to PCP is a function of external pH. Lowering pHe increases the response. The response has an absolute requirement for external Ca 2+ and is a sigmoidal function of external Ca 2+ concentration. Since treatment of these fibers with PCP in high concentration leads to prompt contraction, experiments were designed to determine whether the observed rise in ouabain-insensitive Na efflux is due to a fall in myoplasmic pCa and whether trigger Ca 2+ originates from the bathing medium. The results obtained show that prior injection of ethylene glycol bis(β-aminoethyl ether) N,N′-tetraacetic acid (EGTA) or 1,2-bis(2-aminophenoxyethane- N,N,N′,N′-tetraacetic acid (BAPTA) leads to a drastic reduction in the response to PCP. They also show that prior external application of verapamil or devapamil stops the response to PCP from occurring. Both Cd 2+ and Co 2+ are also effective but only temporarily. Last, the effects of ryanodine and 8-( N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) were tested, since the former is known to block the sarcoplasmic reticulum Ca 2+ release channel, and the latter to impair the action of agents known to release Ca 2+ from internal depots. Both ryanodine and TMB-8 are found to reduce the response to PCP. Taken together, these observations support the hypothesis that PCP stimulates the ouabain-insensitive Na efflux by increasing the internal free Ca 2+ and that the increase in internal Ca 2+ is due to the entry of trigger Ca 2+ from the outside via Ca 2+ channels, as well as release of Ca 2+ is due to the entry of trigger via its channel. They also indicate that the efficacy of PCP depends on the 5 Cl atoms present in its aromatic ring and pH e.