Aspects of comparative neuromuscular physiology and pharmacology of black bass (Micropterus salmoides) as evidenced by the action of diisopropylfluorophosphate Public Deposited

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  • Neuromuscular physiology has been extensively investigated in several groups of vertebrates excluding fish. To understand the nature of neuromuscular transmission in this group, the response of the pectoral fin abductor muscle of black bass (Micropterus salmoides) to nerve stimulation under the influence of an anticho1inesterase agent was studied. Diisopropylfluorophosphate (DFP), a prototype organophosphate, was used because of its well known anticholinesterase activity. Initial studies indicated that DFP did not cause modification of indirectly stimulated twitch or tetanic activity of the pectoral fin muscle even with intravenous administration of 200 mg/kg which was lethal to the fish within 30 minutes. DFP doses were subsequently increased, udder atropine protection, to 600 mg/kg, but no modification of muscular response was observed through periods of up to 110 minutes. Three possible explanations within the framework, of classical cholinergic theory were suggested for the lack of DFP inhibition of the muscle response: 1) DFP was not distributed to the neuromuscular area in concentrations sufficient for AChE inhibition, 2) DFP was distributed to the neuromuscular area but did not inhibit AChE, and 3) DFP was distributed to the neuromuscular area and did inhibit AChE but did not result in alteration of the muscular response. The validity of these explanations was tested in two subsequent phases of experimentation. The next objective was to provide information on the in vitro occurrence and relative characteristics of cholinesterase enzymes presumed to be a component of the pectoral fin motor unit. Acetylcholinesterase (EC 3.1.1.7) was identified in muscle homogenates. Kinetics as well as sub- . strate and inhibitor specificites suggested that the enzyme, had much the same character as that isolated from other vertebrate sources. Furthermore the enzyme was capable of complete inhibition by DFP in vitro. The final objective was to evaluate the significance of AChE to junctional transmission in the pectoral muscle of bass by establishing in vivo, first, the occurrence of enzyme inhibition by DFP and, second, the correlation between muscular response and enzyme inhibition. Dose response curves for inhibition of AChE by DFP were established indicating DFP had access to and did. inhibit AChE in vivo. Modification of the muscular response was observed with high doses of DFP and the severity of modification appeared to be correlated to AChE inhibition. The modification, however, was not typical of cholinergic systems in that the ability to produce and maintain a tetanus was never abolished even with nearly complete inhibition of AChE. It was concluded on, the basis of this information that AChE is not as important for, neuromuscular transmission in black bass as has been established for other vertebrates. The results have also pointed to some apparent contradictions between the character of the muscular response in these preparations and the response of typical vertebrate muscle preparations. These observations substantiate the third of the pos sible explanations for the lack of DFP modification of neuromuscular transmission in the pectoral fin preparation of the black bass, and suggest some divergence from the classical cholinergic concept in that preparation.
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