Is hypercalcemic diet a possible antidote to oral contraceptive-induced hypertension?

Administration of oral contraceptive (OC) has been associated with body fluid retention and in high doses over a long period, promotes hypertension. This present investigation tests the hypothesis that the dietary calcium supplementation increases salt and water excretion in OC (norgestre/ethinylestradiol) treated 32 female albino rats randomly distributed into four (1-4) groups of 8 rats each: Control, OC-treated, OC-treated+ Calcium diet fed and Calcium diet fed only respectively. OC was administered to the appropriate groups by gavage. Experimental diet contained 2.5% calcium supplement. Plasma and urinary [Na+] [K+] were evaluated after 8 weeks of experimentation by flame photometry and plasma [Ca2+] by colorimetric method. OC-treatment induced a significant fall in urinary [Na+]. Water excretion was significantly reduced in these animals (control, 3.1±0.56 Vs OC-treated rats, 1.47±0.16). OC-treated rats had significantly higher plasma [K+] compared to control rats. Calcium supplementation induced increases in plasma [Na+], [K+] and augmented urinary Na+ excretion (OC-treated + Ca2+ diet Vs OC-treated only). Compared with the control rats, high Ca2+ diet fed rats exhibited significant increases in plasma [Na+] and [K+] accompanied by significant decreases in urinary H20 excretion. These results strongly suggest that high dietary Ca2+ supplementation increases salt and water excretion in OC-treated rats and potentially moderates fluid retention and blood pressure in these animals, and may be of clinical significance in OC-induced abnormal fluid retention and perhaps OC-induced hypertension.

Effect of caffeine on the risk of coronary heart disease- A re-evaluation

The effect of caffeine intake on the risk of coronary heart disease was studied. Twenty-one rats used were randomly divided into three experimental groups, the first group served as the control while the second and third groups were administered caffeine orally at doses of 10mg/kg body weight and 20mg/kg body weight respectively for fourteen days. Caffeine, at 10mg/kg body weight, significantly increased (P<0.05) serum LDL- cholesterol concentration and coronary heart disease risk ratio while it significantly reduced (P<0.05) serum triacylglycerol concentration when compared with controls. At 20mg/kg body weight, caffeine significantly increased (P<0.05) coronary heart disease risk ratio while it significantly reduced (P<0.05) serum HDL-cholesterol concentration and serum triacylgycerol concentration when compared with controls. No dose response effect was observed possibly suggestive of a threshold effect. These results suggest that caffeine predisposes consumers of caffeine containing beverages to coronary heart disease.

Modulation of intrinsic prothrombin activation by fibrinogen and fibrin I

Recent observations raise the possibility that by binding thrombin, fibrin(ogen) could modulate prothrombin activation, perhaps by modulating thrombin-mediated activation of factors VIII and V. Factors VIIIa and Va are the cofactors of intrinsic tenase and prothrombinase, respectively. This study compared the profile of prothrombin activation in pooled normal plasma, defibrinated pooled normal plasma, and pooled normal plasma containing fibrin I. Prothrombin activation was initiated with three stimuli, as follows: (1) addition of a suspension of thrombin, CaCl2, and coagulant phospholipids to plasma; (2) contact activation followed by recalcification of plasma; and (3) addition of a suspension of crude rabbit brain tissue factor and CaCl2 to plasma. Each plasma sample contained 2 mmol/L gly-pro-arg-pro, which prevents the polymerization of fibrin. Prothrombin activation, indexed as the concentration of prothrombin activation fragment 1 + 2 produced, was quantitated by enzyme-linked immunosorbent assay. Fibrinogen significantly delayed the onset of prothrombin activation initiated by thrombin or by contact activation but not that initiated with tissue factor. In contrast, fibrin I in solution accelerated prothrombin activation initiated with thrombin or by contact activation. Although fibrinogen had no effect on intrinsic activation of factor IX or factor IX activation initiated by adding thrombin to plasma, it inhibited factor X activation initiated with either stimuli. We therefore conclude that fibrinogen can inhibit the activation of factor X and prothrombin. Second, fibrin I accelerates prothrombin activation. Fibrinogen and fibrin, therefore, have the potential to bioregulate blood coagulation.

Bioregulatory roles for fibrin(ogen) on blood coagulation

A high plasma fibrin(ogen) concentration is a known risk for thrombosis and cardiovascular disease. Because fibrin(ogen) binds alpha-thrombin with high affinity, it has potential to modulate the interactions of the enzyme with its inhibitors and other substrates in plasma. In particular, fibrin moderates the inhibition of thrombin by antithrombin III and antithrombin III/heparin. Recently, we have demonstrated a novel regulatory role for fibrin(ogen) on plasma coagulation. A push-pull hypothesis is proposed for bioregulation of coagulation by fibrin(ogen). The experimental work which led to the formulation of this hypothesis is outlined.

Fibrin moderates the catalytic action of heparin but not that of dermatan sulfate on thrombin inhibition in human plasma

Three recent studies have reported that fibrin in solution significantly inhibits the ability of heparin to catalyze the inhibition of thrombin by antithrombin III. In addition, heparin inhibits the release of fibrinopeptide A by clot-bound thrombin less effectively than it inhibits the release of fibrinopeptide A by thrombin in solution. We have also reported that dermatan sulfate, which catalyzes thrombin inhibition by heparin cofactor II, inhibits thrombus growth in rabbits more effectively than heparin. Because the results of these studies suggest that fibrin inhibits the reactivity of thrombin with antithrombin III-heparin but not with heparin cofactor II-dermatan sulfate, we compared the relative catalytic effects of heparin and dermatan sulfate on thrombin inhibition in plasma, both in the presence and absence of fibrin. We quantitated the rates of thrombin inhibition by antithrombin III and heparin cofactor II by specific enzyme-linked immunosorbent assays. When it was generated, fibrin was kept in solution by adding 2 mmol/L Gly-Pro-Arg-Pro to plasma. Fibrinogen-fibrin reduced the reactivity of thrombin with plasma antithrombin III, both in the presence of and in the absence of heparin. In contrast, the catalytic action of dermatan sulfate on thrombin inhibition by plasma heparin cofactor II was unimpaired by fibrinogen-fibrin. Based on the ability of dermatan sulfate to inhibit thrombus growth in rabbits, failure of fibrinogen-fibrin to moderate the catalytic action of dermatan sulfate may account for its greater antithrombotic effectiveness relative to that of heparin.