Increased cardiovascular responsiveness to central cholinergic stimulation in the genetically epilepsy-prone rat

Soman-induced hypertension in conscious rats is mediated by prolonged central muscarinic stimulation

The acetylcholinesterase inhibitor, soman, induces marked and sustained hypertension and tachycardia associated with a convulsive syndrome in rats. The aims of the present study were to distinguish between the cardiovascular and convulsant effects of soman and to determine whether the maintenance of the soman-induced hypertension and tachycardia depends solely on a central muscarinic effect. To this end, using a computerised analysis of blood pressure (BP) in conscious freely moving rats, we examined the consequences on the increase in mean BP (MBP) and heart rate (HR) induced by soman (60 micrograms/kg, i.v.) of 1) a pre-treatment with the anticonvulsant drug diazepam (3 mg/kg, i.v.) and 2) atropine sulphate (10 mg/kg, i.v.) administered 10 or 60 min after the intoxication. Pretreatment with diazepam prevented the convulsions, assessed by electroencephalogram (EEG) recording, but modified neither the magnitude nor the kinetics of the pressor and tachycardic effects of soman (delta MBP = 74 +/- 2 and 73 +/- 5 mmHg, delta HR = 69 +/- 10 and 79 +/- 7 bpm, maximum MBP = 186 +/- 3 and 182 +/- 6 mmHg, maximum HR = 545 +/- 9 and 522 +/- 16 bpm in solvent- (n = 8) and diazepam- (n = 8) pre-treated rats, respectively). Whatever its time of administration, atropine sulphate fully and immediately reversed the rise in BP induced by soman. The soman-induced tachycardia was also suppressed by atropine administered 10 min after soman whereas it persisted when atropine was injected 60 min after the intoxication. These results show that the cardiovascular effects of soman can occur independently of the convulsive syndrome and that the maintenance of the soman-induced hypertension depends entirely on a permanent central muscarinic stimulation.

Interactions between endothelial secretogogues

Among endothelial secretogogues prostacyclin (PGI2), nitric oxide (NO) and tissue plasminogen activator (t-PA) play a crucial role in maintaining thromboresistance, tone and structure of the vascular wall. Most receptor agonists, such as B2 kinin receptor agonists, or shear force produce a coupled release of all three secretogogues, and therefore interactions between them are to be expected. Essentially, PGI2 is a platelet suppressant, NO a vasodilator and t-PA a fibrinolytic agent. These and other properties of endothelial secretogogues supplement each other in protecting the cardiovascular system from injuries. It is not surprising that disturbances of the secretory function of endothelial cells are associated with atherosclerosis, diabetes, thrombosis or hypertension. Traditionally, PGI2, NO, t-PA or their substitutes are used individually for the treatment of peripheral arterial disease, angina pectoris or acute myocardial infarction. In light of recent findings, their joint administration can be advocated. For instance, NO donors will potentiate platelet-suppressant action of PGI2 analogues, whereas exogenous PGI2 or TXA2 synthase inhibitors (i.e. following increase in endogenous PGI2) will abolish a paradox of prothrombotic action of t-PA or streptokinase. The replacement therapy with PGI2, NO or t-PA should match as closely as possible the physiologically coupled release of these secretogogues.

Endothelial control of coronary flow in perfused guinea pig heart

In perfused isolated guinea pig hearts reactive hyperemia (RH) was induced by occlusion of coronary flow for periods ranging from 1-60 s. RH was hampered by 100-60% in the presence of an inhibitor of NO synthase, NG-nitro-L-arginine (100 microM) and, to a lesser extent (up to 35%), by an antagonist of adenosine receptors, 8-phenyltheophylline (10 microM). An inhibitor of PGH synthase, indomethacin (5 microM), did not affect RH. During RH the heart generated prostacyclin, nitric oxide, and adenosine as indicated by the appearance of 6-keto-PGF1 alpha, cyclic GMP, urate, inosine, hypoxanthine and xanthine in the perfusate. Out of these factors only NO and adenosine were responsible for RH. NO was responsible for RH which was evoked by short-term (1-10s) coronary occlusion, whereas concurrent efforts of NO and adenosine were required to maintain RH that followed longer (20-60s) periods of interruption of coronary inflow. Thus, in the investigated system nitric oxide and adenosine but not prostacyclin can be considered as the mediators of myocardial reactive hyperemia.