Effects Of 2-Methvl-3-~ ropvnvlauinazolin-4-( 3 H )-one On Vascular Reactivitv In Isolated Porcine Tail Arteries

The vascular effects of 2-methyl-3-propynylquinazolin-4-(3H)-one (QUIN) have been studied on isolated porcine tail arteries. QUlN had no effect on resting tension but relaxed, dose-dependently, arteries precontracted with noradrenaline or high-K' in the order: NA > high-K'. Also, QUlN inhibited both intracellular (ICD) and extracellular (ECD) Ca2'-dependent contractions in the order: ICD > ECD The results suggest that QUlN interferes with vascular ca2' mobilization.


Introduction
Quinazolinones are a large group of heterocyclic nitrogen compounds [ I , 2 1 with a wide range of pharmacological activities.Dtfferent derivatives have been reported to have cardiotonic, antihypertensive, antiantrythmic, vasodilator and lipdlowering properhes [3, 41.In a recent study by Ryu et al [5], a 6-(substrtutedphenyl)-amino-5,8-quinazolinedione was reported to have potent inhibitory effect on endothelium-dependent vasorelaxation.
In view of the paucity of information on the cardiovascular m o n s of quinazolones, the goal of the present study was to investigate some vascular effects of a quinazolone derivative: 2-methyl-3-propynylquinazolin4(3H)-0ne (QU I N) in porcine arterial smooth muscle preparations.
Each arterial ring was suspended between a stainless steel hook and an isometric force transducer (Grass FT.03), in a 20ml organ bath containing PSS. Contracbons were displayed on a Grass Model 7 4-channel polygraph.
The PSS was maintained thermostatically at 37OC, pH 7.4 and bubbled continuously wtth 95% 02, 5% C02 gas mixture, under an initial tension of l g .The rings were allowed to equilibrate for 1 hour before the start of experiment.Following the equilibration period, the rings were stimulated twice using 80mM K' PSS (wrth rinses and recovery in-between the stimulations).Subsequent contradons during the experiment, were compared wrth this initial 80mM K' contraction.

Protocols Effect of QUlN on basdine tension:
Arterial rings were exposed to cumulative concentrations of QUIN, to examine the possible effect on baseline tension.

Relaxant Meet of QUIN :
Arterial rings were precontracted using 2 .3 ~1 O -~M noradrenaline or 40mM K* (EC7*: concentration producing 70% of maximun contraction).When the contractions were stable, QUlN was added to the bath, cumulatively (a higher concentration of QUIN was added when the response to the previous concentr&on was steady).

Mbct of QUlN on lntra-and Extmcdlular w-dependent Co-ons
The procedure employed to assess the effect of QUIN on NA-induced phasic (intracellular) and tonic (extracellular) calciumdependent contractions is as previously described [6,7,8]: maximum contractions to NA (1 x104M) were obtained in normal PSS and following 30 min exposure to a nominally calcium-free PSS.
Following the phasic (intracellular Cadependent) contraction, restoration of calcium in the PSS resulted in a tonic (extracellular Cadependent) contraction.To assess the influence of QUlN on both components of contraction, varying concentrations of QUlN were applied simultaneously with the change to Ca-free PSS and maintained throughout the duration of both components of contraction.

Effect of drugs on QUINinduced relaxation:
The influence of various pharmacologic antagonists on QUIN-induced relaxation was examined by estimating the magnitude of the relaxation response induced by 5 x 1 0 -~~ QUlN first, in the absence and following application of a particular antagonist 20min prior to 2 .3 ~1 O -~M NA precontracbon (the antagonist remained in the bath for the duration of the response to QUIN).
The following drugs were used: Noradrenaline bitartrate (Levophed, Stirling Drug Inc.); Cimetidine, Indomethacin, Propranolol, Ouabain (Sigma, UK).The drugs were prepared freshly by dissolving in distilled water.Ca-free PSS contained no added CaCI2 with or &out l.OmM EGTA.High-K' PSS was prepared by equimolar substitution of NaCl with KCI.All chemicals were of analytical reagent grade.

statisb-cs
Values are presented as means k standard error of the mean (S.E.M.) and n represents the number of rats from which tissues were obtained.Comparisons were made where appropriate, by using the Student's t-test ('Microcal Origin' software).A p value less than 0.05 was taken to denote statistical significance in all cases.

Discussion
We have observed, in the present study, that 2-methyl-3-propynylquinazolin-4- release mechanisms and may also provide an explanation for the greater inhibitory effect of QUlN in NA-precontracted rings discussed earlier (Fig. 1).
The lack of effect of indomethacin, ouabain, propranolol and cimetidine on QUIN-induced relaxation of NA-precontracted arterial rings suggests the noninvolvement of cyclooxygenase stimulation, Na'-K' ATPase enzyme activation, Padrenergic stimulation and histamine Hz-receptor activation as possible mechanisms of QUlN adon.
In conclusion, we report that 2-methyl-3-propynylquinazolin4(3H)-one (QUIN) elicits relaxation of vascular smooth muscle of the porcine tail artery by mechanisms associated with interference with ca2' supply, particularly, from the intracellular pool.
The following experimental protocols were examined: (a) Effect of QUlN on baseline tension (b) Relaxant effect of QUlN on noradrenaline or potassium-induced precontraction (c) Effect of Pharrnacologic antagonists on QUIN-induced relaxation and (d) Effect of QUlN on intra-and extra-cellular ca2'-dependent contractions.