Evaluation of the hypolipidemic activity of xenalipin in experimental animals.

Xenalipin (4'-trifluoromethyl-2-biphenylcarboxylic acid) is a chemically novel compound which has been found to be an effective hypolipidemic agent in two animal species. Significant reductions in serum cholesterol and triglycerides were observed in cholesterol-cholic acid-fed rats following oral doses of 10-30 mg/kg/day. Xenalipin was considerably more potent than clofibrate, nicotinic acid, and cholestyramine in the same model. Lipoprotein analysis showed that xenalipin reduced cholesterol and protein content in very low density lipoprotein (VLDL), intermediate density lipoprotein (IDL), and low density lipoprotein (LDL). Triglycerides were reduced in VLDL and IDL. Xenalipin was also effective in reducing serum cholesterol and triglyceride concentrations in normocholesterolemic rats. In diet-induced hypercholesterolemic African green monkeys, xenalipin at oral doses of 15-60 mg/kg b.i.d. reduced serum LDL-cholesterol concentrations. These results suggest that xenalipin has a profile of activity which would be beneficial in therapy for hyperlipidemia.

Urinary and renal tissue kallikrein in the streptozocin-diabetic rat.

The renal kallikrein-kinin system is thought to participate in blood pressure regulation and displays abnormalities in human hypertension, as well as in many animal models of hypertension. Urinary excretion and tissue levels of renal kallikrein were measured in streptozocin (STZ)-diabetic rats in relation to blood pressure, glycemia, and insulin treatment. In study 1, STZ-diabetic rats with marked hyperglycemia showed reduced kallikrein-like esterase excretion, compared with control rats, when first measured after 7 days of diabetes (9.9 +/- 2.5 versus 17.5 +/- 2.4 EU/24 h, P less than 0.05). This difference increased with time and, after 210 days, urinary esterase excretion in diabetic and control rats was 6.7 +/- 2.1 and 39.0 +/- 6.0 EU/24 h, respectively (P less than 0.001). Urine kallikrein, measured by radioimmunoassay, was similarly reduced in diabetic rats (40.4 +/- 8.0 versus 88.0 +/- 6.5 micrograms/24 h, at 30 days, P less than 0.001). At 120 days, systolic blood pressures were elevated in diabetic rats (P less than 0.05), and at 180 days over 60% of the diabetic rats had pressures above the highest pressures of control rats. In study 2, STZ-diabetic rats were treated with insulin for 2 wk (2 U NPH at 0800 h, or 2 U NPH at 0800 and 1600 h). In the single-dose group, with hyperglycemia similar to that of diabetic rats in study 1, kallikrein excretion was reduced as early as day 2, compared with nondiabetic rats (56.0 +/- 6.1 versus 109 +/- 9.4 micrograms/24 h, respectively, P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Renal kallikrein in human and experimental diabetes mellitus.

Urinary kallikrein excretion was studied in a group of insulin-dependent diabetic patients in relation to glycemic control, and both urinary and renal tissue levels of kallikrein were measured in untreated and insulin-treated streptozotocin-diabetic rats. While on a fixed normal sodium diet, diabetic patients in poor glycemic control (HbA1c greater than 11%) showed elevated urinary kallikrein, compared to patients with HbA1c less than 11% or normal subjects. Strict glycemic control lowered kallikrein excretion in diabetic patients. In contrast, severely hyperglycemic diabetic rats had reduced urinary and renal kallikrein levels, compared to nondiabetic control rats, and the reduction in urinary kallikrein preceded the development of systolic hypertension. Insulin treatment normalized urinary kallikrein in diabetic rats, but resulted in increased renal kallikrein levels compared to nondiabetic rats. The differences in kallikrein abnormalities observed in diabetic patients and a diabetic animal model may possibly be related to renal hemodynamic changes in clinical and experimental diabetic states.

Characterization of the thrombin-induced contraction of vascular smooth muscle.

Purified human alpha-thrombin induced a sustained contraction of isolated rabbit aorta and dog coronary arteries. These vascular tissues also exhibited a refractoriness towards a second thrombin exposure. The extent of tachyphylaxis exhibited by the aorta correlated with the initial concentration of thrombin and the length of time the tissue was exposed to thrombin. The thrombin-induced contraction in the aorta was not blocked by phospholipase or cyclooxygenase inhibitors, but it was inhibited in the presence of hirudin, heparin, nitroglycerin, and nitroprusside. Nitroglycerin, nitroprusside, and hirudin also inhibited the contraction in the dog coronary artery. Ca++ channel blockers did not inhibit the thrombin-induced contraction in the coronary artery, although a small inhibition was observed in Ca++-free media. In both tissues, equivalent contractile responses were obtained using equimolar quantities of beta-, tetranitromethane-, and alpha-thrombin, even though the latter's coagulant activity was 30-40 times that of the modified thrombins. However, if the catalytic activity of thrombin was inhibited by modification with Tos-Lys-CH2Cl, hirudin, or heparin/antithrombin III, the vasoconstrictor activity was also lost. These studies suggest that alterations of the fibrinogen-binding site do not affect the contractile activity of thrombin. The contraction may be the result of a proteolytic interaction of the active site of the enzyme with vascular smooth muscle.

Generation of a vasoactive substance in human plasma during coagulation. Evidence of thrombin-induced contraction of rabbit aorta and dog coronary artery.

A stimulant of vascular smooth muscle contraction was generated in fresh, citrated human plasma during activation of the clotting system. Plasma, exposed briefly to thromboplastin and Ca++, induced a contraction of isolated rabbit aorta and dog coronary arteries that was slow in development and persisted after washout. The contractile activity was not blocked by phenoxybenzamine, atropine, or angiotensin inhibitor, but was blocked when heparin or hirudin was incubated with the plasma. The contractile stimulant produced in the plasma was short-lived (less than 3 min) and paralleled the appearance of thrombin in plasma. Purified human alpha-thrombin also induced a sustained contraction in these blood vessels that was not inhibited by phenoxybenzamine, atropine, or angiotensin inhibitor, but was blocked by hirudin. Partial relaxation of the thrombin-treated blood vessel was achieved by the addition of heparin. These results suggest that this vasoactive component of thromboplastin-activated human plasma is alpha-thrombin. Because of its potent and persistent effects, thrombin-induced vasospasm may be an important mechanism in the etiology of ischemic heart disease.

Species specificity of the platelet responses to 1-0-alkyl-2-acetyl-sn-glycero-3-phosphocholine.

A semi-synthetic 1-0-alkyl-2-acetyl-sn-glycero-3-phosphocholine (alkylacetyl-GPC) was shown to be highly species selective in its capacity to cause platelet aggregation and serotonin release. No effects were elicited on the rat or mouse platelets while platelets from human, dog, cat, rabbit, guinea pig and horse were highly sensitive to alkylacetyl-GPC. The hypotensive activity in the rat was not associated with thrombocytopenia. Preliminary evidence suggested that the inability of platelets of the rat and mouse to respond to alkylacetyl-GPC was not due to a difference in plasma inactivation of the substance but due to a difference in platelet responsiveness per se. The data also support the concept that the potent hypotensive property of this substance readily observed in the rat, is a result of an effect which is platelet-independent.

Synthesis of prostaglandin 6-keto F1alpha by cultured aortic smooth muscle cells and stimulation of its formation in a coupled system with platelet lysates.

Lysed aortic smooth muscle cells, when incubated with [14C] arachidonate, synthesized only one radioactive product, which was identified as 6-keto-PGF1alpha. Formation of this product from smooth muscle cell lysates was stimulated when human platelet extracts were added to the system, and further stimulation was observed when imidazole, a selective inhibitor of thromboxane synthesis, was added to this coupled system. These observations indicate that the cyclooxygenase of the smooth muscle cells was rate-limiting, that the prostacyclin synthetase of these cells can utilize endoperoxides produced by platelets, and that blocking of thromboxane synthesis might, under certain conditions, shunt arachidonate metabolism toward prostacyclin formation.

Occurrence and function of cyclic nucleotides in blood vessels.

The literature concerned with studies of the occurrence and function of the cyclic nucleotides in blood vessels is reviewed. Emphasis is placed on the critical evaluation of the evidence which relates to the hypothesis that cyclic nucleotides meditate the effects of drugs and neurotransmitters on vascular contractility. The hypothesis that cyclic AMP mediates vasodilation, especially that induced by beta-adrenergic relaxation, is supported by many experimental approaches, but it is concluded that the evidence remains unconvincing based on the criteria established for such a mediator role. Possible sites of action of cyclic AMP are discussed. The demonstrated action of cyclic AMP on vascular membrane electrophysiology and calcium ion pumps are reviewed as possible causes of relaxation. The role of both nucleotides in vascular disease, especially hypertension is discussed. Finally the needs for further research in this area are suggested.

Effects of bretylium on rat cardiac muscle: the electrophysiological effects and its uptake and binding in normal and immunosympathectomized rat hearts.

Bretylium produced electrophysiological effects on both rat atrium and ventricle in vitro at concentrations ranging from 2 times 10- minus 5 to 10- minus three M. Those effects included lengthening of action potential duration and effective refractory period; increasing effective refractory period/action potential duration; decreasing dv/dt of phase zero of the action potential and suppressing the action potential amplitude and overshoot. These effects, which could serve as a basis for the antiarrhythmic action of bretylium, were observed also in hearts from immunosympathectomized rats confirming a direct effect of this drug on the electrical properties of the cardiac muscle cells. In vivo and in vitro exposure of the myocardium to 14-C-bretylium showed that this drug is concentrated in cardiac ventricle and that this concentrating ability of the heart may be responsible for attaining effective antiarrhythmic concentrations in the myocardium at low plasma concentrations of the drug. Uptake of bretylium by the sympathetic nerves never amounted to more than 15% of the total bretylium binding by the cardiac ventricle and this neuronal uptake became insignificant compared to total bretylium uptake at concentrations greater than 10- minus 6 M. Subcellular distribution of the bretylium bound to the cardiac ventricle from immunosympathectomized rats suggested a binding to plasma membranes. Efflux studies indicate that this binding was tight, although reversible. These results indicate that underlying the antiarrhythmic effects of bretylium is an accumulation of the drug by cardiac muscle cells and a direct effect of the drug on the electrical properties of the cardiac muscle membrane independent of any action on the adrenergic neuron.