Disparity between blood pressure and PRA inhibition after administration of a renin inhibitor to anesthetized dogs: methodological considerations

A dissociation between changes in blood pressure (BP) and plasma renin activity (PRA) has been noted after administration of renin inhibitors. In the present study, the renin inhibitor PD 132002 was given to salt-deplete, anesthetized dogs. PRA was measured at pH 6.0 by a conventional angiotensin I (ANG I) RIA method (PRA-C) and by an ANG I antibody-trapping RIA method (PRA-AT) performed at pH 7.4. PD 132002 at 0.01, 0.1, 1, and 10 mg/kg IV, reduced BP by 3 +/- 2, 9 +/- 2, 24 +/- 4, and 39 +/- 4 mm Hg, respectively, (baseline of 136 +/- 8 mm Hg, N = 5), when infused IV over 30 minutes with a 30 minute recovery between doses. The BP response at 10 mg/kg equaled that of saralasin (20 micrograms/kg/min IV). PRA-AT (baseline of 20 +/- 6 ng ANG l/ml/hr, N = 4) was inhibited by 0%, 28% +/- 12%, 75% +/- 10%, and 97% +/- 1% at 0.01, 0.1, 1, and 10 mg/kg, respectively. Plasma concentrations of immunoreactive ANG II were also reduced dose-dependently and paralleled changes in BP. In contrast, PRA-C (baseline of 13 +/- 4 ng ANG l/ml/hr, N = 4) was inhibited by 82% +/- 8% at 0.01 mg/kg and by > 98% at higher doses. After a single dose of PD 132002 at 10 mg/kg infused over 30 minutes, BP recovery paralleled changes in immunoreactive ANG II and PRA-AT, yet PRA-C inhibition showed no recovery over the same time course. Our data support the conclusion that BP relates better to PRA-AT than PRA-C. Thus the dissociation sometimes observed in studies with renin inhibitors between changes in BP and PRA may be attributed to the assay used to determine PRA.     

Selectin- and integrin-mediated T-lymphocyte rolling and arrest on TNF-alpha-activated endothelium: augmentation by erythrocytes

The adhesive and hemodynamic forces that lead to lymphocyte rolling and arrest on activated endothelium and the biophysical role of various adhesion molecules and blood elements in this process are poorly understood. By quantifying their behaviour both in vivo and in vitro, we show here that erythrocytes facilitate selectin- and integrin-mediated rolling and binding of T-lymphocytes on tumor necrosis factor alpha-activated endothelium. The relative contribution of selectins and integrins to this process can be distinguished by using a simple mathematical expression of lymphocyte capture within the range of physiological shear stress. The need for selectin participation in lymphocyte capture increases with shear stress (> 1 dyn/cm2), and both beta 1 and beta 2 integrins act in synergy to produce adhesive drag on captured cells. These findings are potentially useful in developing strategies for intervening with T-cells in a variety of normal and pathological responses as well as for the delivery of genetically modified T-cells to their targets in vivo.     

The activation of prothrombin by the prothrombinase complex. The contribution of the substrate-membrane interaction to catalysis

The conversion of prothrombin to thrombin requires the cleavage of two peptide bonds and is catalyzed by the prothrombinase complex composed of factors Xa and Va assembled on a membrane surface. Presteady-state kinetic studies of the effects of membranes on the proteolytic reaction were undertaken using model membranes composed of phosphatidylcholine and phosphatidylserine (PCPS). The concentration of PCPS was varied to alter the concentration of free phospholipid available for substrate binding without influencing the concentration of membrane-assembled prothrombinase. In fluorescence stopped-flow measurements, increasing concentrations of PCPS resulted in an increase in the rate of product formation. Assessment of bond cleavage by sodium dodecyl sulfate-polyacrylamide gel electrophoresis following rapid chemical quench using 125I-prothrombin revealed that the activation reaction proceeded through the ordered cleavage at Arg323-Ile324 followed by cleavage at Art274-Thr275 at all concentrations of PCPS. Increasing the PCPS concentration resulted in a large increase in the Arg323-Ile324 cleavage reaction with a much smaller effect on the subsequent cleavage at Arg274-Thr275, thereby leading to an increase in the extent of accumulation of the intermediate, meizothrombin. Fluorescence stopped-flow and rapid chemical quench measurements were also conducted using prethrombin 2 plus fragment 1.2 or meizothrombin as substrates to assess the influence of PCPS on the individual cleavage reactions. The rate of cleavage at Arg323-Ile324 by prothrombinase was increased approximately 60-fold with increasing PCPS, whereas the cleavage at Arg274-Thr275 was increased by a factor of approximately 5. These differential effects of PCPS on the two cleavage reactions adequately explain changes in the extent of accumulation of meizothrombin during prothrombin activation. Proteolytic removal of the membrane binding fragment 1 domain of the substrates, meizothrombin and prethrombin 2-fragment 1.2, had no effect on the cleavage at Arg274-Thr275 at saturating PCPS but completely eliminated the membrane-dependent rate enhancement for cleavage at Arg323-Ile324. Thus, membrane binding by the substrate is essential for the first cleavage reaction at Arg323-Ile324, which leads to the conversion of prothrombin to meizothrombin. In contrast, the substrate-membrane interaction mediated by the fragment 1 domain has no detectable effect on the second cleavage reaction at Arg274-Thr275 which is required for the conversion of meizothrombin to thrombin.     

Visual performance after photorefractive keratectomy. A prospective study

OBJECTIVE: To prospectively examine the effect of excimer laser photorefractive keratectomy (PRK) on best-corrected visual performance using psychophysical tests that were likely to be more sensitive to image degradation than high-contrast Snellen visual acuity. DESIGN: Prospective cases series. PATIENTS: A cohort of 18 subjects with an average of -5.08 diopters (D) of myopia (SD = +/- 1.63 D) was tested before PRK and at 3, 6, and 12 months after PRK. INTERVENTION: Photorefractive keratectomy was performed using a laser (Excimed UV200, Summit Technology, Waltham, Mass) and a polymethylmethacrylate mask; a 5-mm ablation zone was used. MAIN OUTCOME MEASURES: Best-corrected high-contrast visual acuity, best-corrected low-contrast visual acuity (18% Weber contrast), and best-corrected letter-contrast sensitivity. Measurements were repeated with dilated pupils and in the presence of a glare source. RESULTS: One year after PRK, the mean best-corrected high-contrast visual acuity was reduced by half a line (P = .01), and the mean best-corrected low-contrast visual acuity was reduced by 1 1/2 lines (P = .002). The losses were somewhat greater when the subject's pupils were dilated and a glare source was used. The reduction in dilated low-contrast visual acuity was positively correlated with the decentration of the ablation zone (r = 0.47), providing evidence of an association between corneal topography and the functional outcome of PRK. CONCLUSION: Low-contrast visual acuity losses after PRK are notably greater than high-contrast visual acuity losses for best-corrected vision. Low-contrast visual acuity is a sensitive measure for gauging the outcome success and safety of refractive surgery.     

Standards of care for treating headache in primary care practice. National Headache Foundation

The following is a summary of guidelines created under the auspices of the National Headache Foundation, in an effort to improve the care of headache patients in primary care practice. The guidelines represent the consensus of an advisory panel of practitioners chosen by the NHF for their expertise in four specialty areas. A complete set of guidelines can be obtained by calling the National Headache Foundation at 1-800-843-2256 or by writing to them at 428 W. St. James Pl., 2nd Floor, Chicago, IL 60614-2750; the cost is $10.     

Progress in the development of a transcutaneously powered axial flow blood pump ventricular assist system

Development of the Jarvik 2000 intraventricular assist system for long-term support is ongoing. The system integrates the Jarvik 2000 axial flow blood pump with a microprocessor based automatic motor controller to provide response to physiologic demands. Nine devices have been evaluated in vivo (six completed, three ongoing) with durations in excess of 26 weeks. Instrumented experiments include implanted transit-time ultrasonic flow probes and dual micromanometer LV/AoP catheters. Treadmill exercise and heart pacing studies are performed to evaluate control system response to increased heart rates. Pharmacologically induced cardiac dysfunction studies are performed in awake and anesthetized calves to demonstrate control response to simulated heart failure conditions. No deleterious effects or events were encountered during any physiologic studies. No hematologic, renal, hepatic, or pulmonary complications have been encountered in any study. Plasma free hemoglobin levels of 7.0 +/- 5.1 mg/dl demonstrate no device related hemolysis throughout the duration of all studies. Pathologic analysis at explant showed no evidence of thromboembolic events. All pump surfaces were free of thrombus except for a minimal ring of fibrin, (approximately 1 mm) on the inflow bearing. Future developments for permanent implantation will include implanted physiologic control systems, implanted batteries, and transcutaneous energy and data transmission systems.