THE EFFECTS OF THIOPENTAL AND PENTOBARBITAL ON VOLUNTARY MUSCLE

¹ Department of Pharmacology, Jefferson Medical College, Philadelphia, Pa.

Thiopental and pentobarbital cause potentiation of the maximal twitch of the dog and cat tibial muscle, thiopental being the more active drug. This effect is possibly mediated through a slowing of the contractile process in the fibers, thereby allowing a larger proportion of the true contraction to be transmitted through the elastic element to the recording device before relaxation begins. The tensions of maximal tetanic contractions are not similarly increased and the ability to maintain the tensions is reduced.

The effects at the neuromuscular junction are mixed. Both barbiturates in small amounts antagonize curare (head drop) and acetylcholine; however, in the innervated tibial preparation, pentobarbital tends to reinforce the effects of both curare and decamethonium while thiopental, except under special conditions, antagonizes both. The barbiturates then appear to manifest two activities or two aspects of one activity: a non-specific antagonism to the effects of end plate drugs and a weak ability to block neuromuscular transmission. Thiopental is more prone to display the first activity and pentobarbital the second.

The intensification by the barbiturates of partial neuromuscular paralysis by depolarizing agents is subject to interpretation as a manifestation of weak curari-form activity.