Effects of high hydrostatic pressure on neuromuscular transmission in shallow-living and deep-living crustaceans

Abstract

The effects of high hydrostatic pressure on excitatory neuromuscular transmission in shallow- and deep-living crustaceans were compared. Pressure caused depression of the amplitude of excitatory junctional potentials (e.j.p.s) at the neuromuscular junction in the shallow-living crab, Libinia emarginata. A pressure of 100 atm depressed the e.j.p. amplitude by about one-half. In the deep- sea crab, Geryon quinquedens, which ranges to a depth of 2000 m (or 200 atm pressure), adaptations to high pressure were observed in two different types of muscle fibers: 1) In fibers with e.j.p.s that showed high levels of facilitation, the magnitude of pressure-induced depression decreased with increasing frequency of nerve stimulation; i. e., there was a pressure-induced increase in facilitation. Also a pressure-induced increase in the duration of the falling phase of the e.j.p. was observed which served to increase the level of depolarization resulting from summation of the e.j.p.s at high frequencies of nerve stimulation. In these highly facilitating fibers the physiologically significant frequencies that cause appreciable contraction are probably high. At high frequencies the pressure-induced increases in facilitation and summation together served to completely counteract the depressive effect of pressure, and the net depolarization attained during a train of nerve stimulation was relatively unaffected by pressures up to at least 200 atm. 2) Fibers with e.j.p.s showing low levels of facilitation may undergo significant contraction at low frequencies of nerve impulses where neither facilitation nor summation play a significant role. The amplitude of e.j.p.s recorded from this fiber-type in the deep-sea crab were, on the average, unaffected by pressures to 200 atm. The e.j.p.s of some of these fibers showed depression, but others were amplified under pressure. The results of experiments with the lobster, Homarus americanus, which ranges to a depth intermediate between Libinia and Geryon were in many respects intermediate between the results obtained with the two species of crab. Studies of the effect of pressure on isometric tension developed by whole muscles in Homarus and Geryon were consistent with the results of the studies of the e.j.p.; pressure depressed the rate of rise of tension in Homarus and had little effect in Geryon. The results of this work provides a physiological basis for the observation that shallow-living animals are generally immobilized by pressures in excess of 200 atm. Experiments were performed in an attempt to elucidate the mechanism underlying the pressure-induced depression of e.j.p. amplitude. Results were suggestive that the depression of e.j.p. amplitude reflects a pressure-induced decrease in the number of quanta of transmitter substance released by the nerve endings.