Gamma-thujaplicin (GT) and beta-hydroxy thujaplicin (BHT) are two of the isopropyl derivatives of tropolone found in the heartwood of western red cedar (Thuja plicata D. Don). Several of the tropolones are effective inhibitors of the enzyme catechol-O-methyltransferase (COMT). In this work, the isolated, spirally-cut rabbit thoracic aorta preparation has been used to study possible COMT inhibition as well as other pharmacological properties of the sodium salts of GT and BHT. It was found that GT, BHT and pyrogallol, a known COMT inhibitor, all potentiated the response of the aortic strip to norepinephrine (NE). However, since it was shown that the chelating agent EDTA also potentiated the NE response, the potentiation by GT and BHT could not be attributed to COMT inhibition without further evidence. GT and BHT were found to have a stimulatory effect of their own on aortic smooth muscle tissue. BHT was a more potent agonist than GT. Phenoxybenzamine blocked the stimulatory effects of BHT and GT. This suggested alpha-adrenergic receptor involvement although the halogenoalkylamine blocking agents are not absolutely specific in their action. Cocaine produces a small potentiation of the contraction produced by GT and BHT. This potentiation suggests the involvement of endogenous NE. Cocaine does not cause relaxation of the GT and BHT responses which indicates that GT and BHT differ in mode and perhaps site of action from tyramine. BHT potentiates the tyramine response on the rabbit aortic strip. This may be due to COMT inhibition, increased NE release or merely additive effects. GT was found to produce relaxation of a histamine-induced contraction. This relaxant effect was not prevented by beta-adrenergic blockade and is likely due to a nonspecific depressant effect by GT. Both GT and BHT have produced non-specific blocking effects against NE and histamine as well as acetylcholine throughout this work. In reserpinized preparations, GT no longer produced a contraction of the aortic strip. Instead, a relaxation below normal tone was produced. It would seem that the presence of endogenous NE is required in the tissue stores before GT can cause the strips to contract. GT causes relaxation of tyramine-induced contractions in reserpinized strips. The isolated rabbit thoracic aorta was found not to be a suitable preparation for the pharmacological investigation of COMT inhibition by GT and BHT due to their own agonistic effects on this tissue as well as their nonspecific chelating properties. However, this tissue was useful in providing information on other pharmacological actions of these compounds. / Medicine, Faculty of / Anesthesiology, Pharmacology and Therapeutics, Department of / Graduate

Diazoxide, a non-diuretic benzothiadiazlne antihypertensive agent, is thought to act directly upon the vascular smooth muscle of the resistance vessels to exert its therapeutic effects in hypertension. Diazoxide may exert its antihypertensive action by antagonizing calcium in vascular smooth muscle. Wohl et al. (1967 and 1968) have suggested such an interaction based on experiments conducted with isolated rabbit aortae. The present experiments were designed to investigate the possible cellular locus of the postulated interaction of diazoxide with calcium using the isolated anterior mesenteric vein of the rabbit as a model of vascular smooth muscle. This vein is spontaneously motile and possesses characteristics similar to those observed for vessels of the microcirculation. Diazoxide at 10ˉ⁴ M inhibited spontaneous motility and its associated membrane electrical activity, and caused hyperpolarization in rabbit anterior mesenteric veins examined with a sucrose gap apparatus. Diazoxide also inhibited spontaneous electrical and contractile activity in guinea-pig taenia coli and in estrogen dominated rabbit uterus. In all these tissues, calcium is believed to play an important role in spontaneous electrical membrane activity. Diazoxide failed to affect contractility, rate of spontaneous contractions, or action potential configurations in isolated rabbit heart, even though the action potential in heart tissues possesses a definite calcium current component. Diazoxide reduced contractions induced in the mesenteric vein by electrical stimulation of the smooth muscle itself or by excitation of the nerve endings within the vein. Various drugs were chosen for their ability to contract the mesenteric vein in different ways. Noradrenaline contracts vascular smooth muscle even when the tissue Is depolarized with ouabain Diazoxide failed to inhibit noradrenaline contractions in the depolarized vein, but showed the characteristics of a competitive inhibitor of noradrenaline in normally polarized veins. Diazoxide was also capable of inhibiting contractions to serotonin and procaine, agents which require membrane polarization to initiate contraction. The inhibitory effect of diazoxide was not observed to be modified in solutions containing high concentrations of calcium. Diazoxide was tested upon the contractile responses to calcium In veins depolarized in K⁺ Ringer solution. Examination of the resultant dose response curves showed that diazoxide inhibited calcium contractions ln a reversible, non surmountable manner. Hydrochlorothiazide had no effect upon calcium induced contractions. Diazoxide antagonizes drug induced contractions only if a polarized membrane is present. Calcium Induced contractions in depolarizing solutions were inhibited in an apparently Insurmountable manner, while drug responses in polarizing solutions were inhibited by diazoxide in a surmountable manner. In addition, action potentials from rabbit heart were unchanged whereas, the apparently calcium spike mediated electrical activity of certain smooth muscles is inhibited. It is concluded that diazoxide affects the membrane of vascular smooth muscle to reduce excitability of the tissue to drugs or electrical stimuli. It is possible that cell membrane bound calcium could be the locus of action of diazoxide and that this agent modifies membrane calcium to cause increased membrane stability. / Medicine, Faculty of / Anesthesiology, Pharmacology and Therapeutics, Department of / Graduate