Reversible inhibition of renal microsome calcium pump by furosemide

The pharmacology and therapeutics of diuretics in the pediatric patient

Selection of appropriate diuretic therapy in children is hampered by a lack of age-specific pharmacokinetic and pharmacodynamic data, especially in premature neonates. Well-designed clinical trials in neonates, infants, and younger children are necessary prerequisites to safer and more efficacious diuretic therapy.

An examination of the ability of inositol 1,4,5-trisphosphate to induce calcium release and tension development in skinned skeletal muscle fibres of frog and crustacea

We have examined the ability of inositol 1,4,5-trisphosphate (InsP3) to cause contractions of mechanically skinned muscle fibres of frog and barnacle. InsP3 (10-500 microM) did not cause any tension development in 25 frog skinned fibres and 26 barnacle myofibrillar bundles, although contractions could be readily evoked by caffeine and by replacement of an impermeant anion by Cl-, treatments known to release calcium from the sarcoplasmic reticulum (SR). Four barnacle bundles did give responses to InsP3. InsP3 did not modify responses to caffeine or calcium-induced calcium release. Free Mg2+ was lowered to 40 microM and 15 mM D-2,3-diphosphoglycerate was added in order to inhibit the possible breakdown of InsP3 by inositol trisphosphatase. Neither measure revealed a response to InsP3. Arsenazo III absorbance measurements failed to detect any binding of Mg2+ (0-0.5 mM) by 0.35 mM InsP3 in our solutions. Inhibitors of SR calcium uptake (cadium, quercetin, furosemide), omission of EGTA from the solution and varying the temperature from 4 degrees to 22 degrees C also failed to reveal a response of frog skinned fibres to InsP3. The nucleotide GTP, which has been reported to enhance InsP3-induced calcium release from rat liver microsomes, had no effect at 50 microM on the response of frog fibres to InsP3. It is concluded that under conditions in which other calcium release mechanisms operate well, InsP3 is relatively ineffective at releasing calcium from the SR in amounts sufficient to induce contraction. Although we have been unable to find evidence to support the proposed role of InsP3 as an essential link in excitation-contraction coupling of skeletal muscle, we cannot entirely reject its role if essential cofactors are lost in the skinned preparations.

Effects of pharmacological and physiological modulators on Ca-ATPase and alkaline phosphatase activities from guinea-pig placenta in vitro

In a search for modulators of Ca-ATPase and AP activities, we examined three pharmacological agents and the cations Ca2+ and Zn2+. Placental Ca-ATPase specific activity was uncompetitively inhibited in vitro by millimolar concentrations of the diuretics ethacrynic acid and furosemide. Cysteine, a sulphydryl donor, partially reversed the ethacrynic acid inhibition but enhanced the furosemide inhibition, indicating that sulphydryl-binding may be part of the mechanism of the inhibition of Ca-ATPase by ethacrynic acid but not by furosemide. In contrast to Ca-ATPase, AP activity was enhanced by both ethacrynic acid and furosemide. Zinc inhibited Ca-ATPase activity at all concentrations tested, but enhanced and, at higher concentrations, inhibited AP activity. The inhibition of AP activity by D-penicillamine was reversed by Zn, supporting the view that this drug acts by chelating Zn which is essential for AP activity. D-penicillamine had no significant effect on Ca-ATPase activity. Calcium activated both enzyme activities but inhibited only AP activity at higher concentrations. These results indicate that placental Ca-ATPase and AP activities may be distinct and dissociable based on responses to various pharmacological and physiological modulators.

Carbon dioxide or bicarbonate ions release Ca2+ from internal stores in crustacean myofibrillar bundles

The paper describes an investigation into the increase in intracellular free Ca2+ and resting tension of barnacle muscle fibers when exposed to CO2. Isometric tension was recorded in isolated myofibrillar bundles prepared from barnacles and crabs. On replacement of a low relaxing bathing solution (free Ca2+: 20 nM) at pH 7.1 with a similar one containing 100% CO2 and 13 mM HCO3-, also at pH 7.1, the bundles developed a phasic contraction, which aequorin experiments confirmed was due to a release of Ca2+ from a store within the bundles. The source of this Ca2+ is tentatively identified as the sarcoplasmic reticulum (SR) for the following reasons: (1) prior exposure to 20 mM caffeine depleted this Ca2+ store, (2) procaine (10 mM) inhibited the response, and (3) the extracellular space or "clefts" and the mitochondria could be eliminated as possible sources. An effect of the CO2 + HCO3- on the free Ca2+/Mg2+ ratio in the bathing solution was excluded as a possible mechanism. The diuretic furosemide (1 mM) enhanced the response to CO2 + HCO3-. Both furosemide and SITS (1--10 mM), by themselves, also released Ca2+ in myofibrillar bundles. A scheme is put forward to explain these results: it is suggested that diffusion of dissolved CO2 into the SR produces an acidification of the SR lumen, which modifies either the Ca2+/-ATPase or the Ca2+-induced release process in such a way to release Ca2+.

Effects of furosemide on neural mechanisms in Aplysia

The effects of furosemide on action potentials and responses to several neurotransmitters have been studied in the neurons of Aplysia. Furosemide (10(-7) and 10(-3) M) does not visibly affect the normal action potential in R15 neurons. However, when TTX (30 microM) is used to block the sodium component in R15, the remaining spike (presumably the calcium component) is increased in amplitude in the presence of furosemide. Furosemide also alters transmitter-induced conductances. Furosemide greatly reduces the amplitude and shifts, in a depolarizing direction, the reversal potential of chloride-dependent responses to gamma-aminobutyric acid (GABA) and acetylcholine (ACh). This suggests that furosemide both blocks the chloride channel and inhibits a chloride pump. ACh-induced sodium responses were also reduced by furosemide but to a lesser extent than chloride responses. The potassium response to ACh and a voltage-dependent calcium response to serotonin were not altered. These results indicate that furosemide could alter synaptic responses both presynaptically by enhancement of calcium flux during the action potential and postsynaptically by blockade of chloride and sodium conductances.