Effects of convulsant barbiturates on vascular smooth muscle

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Abstract
The convulsant barbiturate, 5-(2-cyclohexylidene-ethyl)-5-ethyl barbituric acid (cheb), produces contraction in rabbit aortic strips. Contractions effected by either cheb or tyramine were preceded by a lag time and both agents induced tachyphylaxis; however, cross-tachyphylaxis could not be demonstrated. Phenoxybenzamine and atropine failed to affect cheb-induced responses, whereas pentobarbitone selectively blocked and also reversed cheb contractions. Prevention, but not reversal, of tachyphylaxis was also accomplished with pentobarbitone. These results suggest that cheb does not act through the release of noradrenaline or acetylcholine; nor does it exert an effect on the receptors for these amines or on those for histamine. Pentobarbitone, however, appears to compete with cheb for common receptors. Another convulsant barbiturate, 5-ethyl-5-(dimethylbutyl)barbituric acid (dmbb), and its optical isomers were also examined. The racemic mixture had no contractile activity, but the (+)-isomer elicited cheb-like effects. The (-)-isomer, on the other hand, was like pentobarbitone in that it antagonized both cheb- and (+)-dmbb-induced contractions. These studies illustrate that convulsant barbiturates are able to stimulate vascular smooth muscle; therefore, it is suggested that the rabbit aortic strip may serve as an in vitro working model for study of the mechanism of action of these drugs in the central nervous system.

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