Tight Chemomechanical Coupling of the F1 Motor Relies on Structural Stability

The F₁ motor is a rotating molecular motor that ensures a tight chemomechanical coupling between ATP hydrolysis/synthesis reactions and rotation steps. However, the mechanism underlying this tight coupling remains to be elucidated. In this study, we used electrorotation in single-molecule experiments using an F₁βE190D mutant to demonstrate that the stall torque was significantly smaller than the wild-type F₁, indicating a loose coupling of this mutant, despite showing similar stepping torque as the wild-type. Experiments on the ATPase activity after heat treatment and gel filtration of the α₃β₃-subcomplex revealed the unstable structure of the βE190D mutant. Our results suggest that the tight chemomechanical coupling of the F₁ motor relies on the structural stability of F₁. We also discuss the difference between the stepping torque and the stall torque.

Pharmacodynamic characterization of posture-related diuretic and saluretic responses to furosemide in humans

To characterize the postural effect on the pharmacodynamic response to furosemide, seven healthy female subjects received the drug (30 mg i.v.), and the diuretic response and urinary drug excretion were monitored either in the supine or upright posture more than 5 h after dose. Six days later the same procedure was repeated, each subject being assigned to the other posture. Diuresis, chlor- and natriuresis were about 50% greater (P less than .01) in the supine than in the upright posture up to 45 min after dose, despite no significant difference in urinary drug excretion between the postures. Creatinine clearance and fractional sodium excretion were also significantly (P less than .05) greater in the supine posture. Pharmacodynamic analysis with the Emax model revealed that the maximum drug response (Emax) to furosemide in the upright posture was significantly (P less than .01) attenuated by about 65% of that in the supine posture, with no significant difference in either ER50 (urinary drug excretion rate producing 50% of Emax) or slope between the postures. Plasma norepinephrine concentration was significantly (P less than .05) greater, whereas urinary kallikrein excretion was significantly (P less than .05) less in the upright posture. There were no significant differences in blood pressure, plasma renin activity and atrial natriuretic peptide as well as in urinary excretion of prostaglandin E2, dopamine and adenosine between the postures.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of nifedipine on urinary concentrating ability: a placebo controlled study

A placebo-controlled, randomized, crossover study was conducted to assess a possible effect of a dihydropyridine Ca-entry blocker, nifedipine, on urinary concentration ability in nine healthy men under a water-deprivated condition. Placebo and nifedipine (20 mg) were orally administered on two separate occasions, at least one week apart, after the urinary osmolarity was stabilized. Urinary osmolarity, osmolar clearance, negative free water clearance, urine volume, urinary solutes (Na, K and urea), creatinine clearance and plasma vasopressin (AVP) were measured during the postdose 3-hour period and compared with those during the respective predose (baseline) period. Urinary osmolarity decreased by nifedipine from 1047.2 +/- 34.4 to 873.0 +/- 38.3 mOsm/kg (mean +/- SEM) at 2 hours postdose (P less than 0.05). Mean % decrease in urinary osmolarity at 1 to 3 hours after nifedipine was significantly (P less than 0.01) greater than after placebo. Urine volume significantly (P less than 0.01) increased from the baseline of 0.49 +/- 0.06 to 1.1 +/- 0.15 ml/min at 2 hours after nifedipine. Relationship between osmolar clearance and negative free water clearance relative to glomerular filtration rate observed during the postnifedipine phase was significantly (P less than 0.01) shifted downward compared with that derived from the pooled data unrelated to nifedipine dosing. No significant drug effect was detected on plasma AVP. Both placebo and nifedipine dosed during the continued water deprivation and stabilized urinary osmolarity condition caused an increase in the urinary excretions of solutes. The results indicate that nifedipine inhibits urinary concentration. This does not appear to be due to the inhibition of AVP secretion from the hypophysis.(ABSTRACT TRUNCATED AT 250 WORDS)