Decamethonium in the Perfused and Immersed Rat Diaphragm

Abstract

1 The water content and mannitol space of rat diaphragms which were perfused through the inferior vena cava was increased compared with immersed diaphragms. The potassium content of both preparations, when expressed in terms of dry weight, was maintained at similar values to that found in vivo. 2 Despite the application of a constant concentration of decamethonium, a steady level of neuromuscular block was not obtained in either the perfused or immersed rat diaphragm. The immersed preparation differed from the perfused preparation in that recovery from paralysis occurred despite the continued presence of the drug. 3 The rate of uptake of labelled decamethonium (100 Mm) at the end-plate region was similar in the perfused and immersed diaphragm. The slopes of the regressions were 0.059 and 0.054 ,ul mg1min-' (based on dry weight) respectively which were not significantly different. This implies that the rate of uptake of the drug at the end-plate is slow and limited by the rate of entry into the fibre rather than by diffusion to the site of entry. IntroductionMethods Creese & Maclagan (1970) have obtained autoradiographic evidence that decamethonium enters muscle fibres in the rat. It has been suaggested that the permeability of this organic ion at the end-plate is low and similar to that of sodium (Creese & England, 1970). If this is the case, then the uptake of decamethonium would be expected to be limited by entry at the cell membrane rather than by diffusion. If however, the permeability of decamethonium approached that of potassium then the uptake would be faster in a perfused muscle than in an immersed muscle because of delays due to diffusion in the latter (Klaus, Luilman & Muscholl, 1960;Keynes, 1954). In the present study, rat diaphragms were perfused in vitro via the inferior vena cava with labelled drug to compare the rate of uptake with that in immersed muscles. In addition, the neuromuscular block produced by decamethonium was recorded in an attempt to confirm previous claims that a steady paralysis can be obtained with depolarizing drugs (Maclagan, 1962; Gibberd, 1966) when applied via the capillaries. The water, potassium and sodium content and also the mannitol space were determined in both preparations. Immersed diaphragmThe left anterior portion of the diaphragm together with its rib was quickly removed, with or without the left phrenic nerve and transferred to a bath (100 ml) for recording of contractions, or to a tube (10 ml) for measurement of uptake of labelled decamethonium. Twitch tension was recorded by means of a strain gauge connected to the tendon by a thin platinum wire (0.4 g). The uptake of the labelled drug was measured in the manner described by Creese & England (1970).After immersion in labelled drug the muscle was 368 P.P.A. HUMPHREY washed in saline for 10 min, attached to a metal strip and frozen on solid carbon dioxide. Perfused diaphlragmThe preparation is similar to that described by Gibberd (1966). The inferior vena cava was cannulated through the right auricle and the e...

Labelled Decamethonium in Cat Muscle

“…Longitudinal sections showed grains all along the fibres, with a concentration in the region of the end-plate and for several hundred microns on either side. There is some indication that decamethonium is entering the cells via the receptor-activated sodium channels; Creese & England (1970) and Humphrey (1975) both found that the permeability of rat muscle to decamethonium was similar to that of sodium and much lower than that of the potassium ion. In addition, tubocurarine prevents both the entry of the drug into the cells and the pharmacological action on the receptors.…”