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Biomedical subjects

A Scarpa

Publications and source records attributed to A Scarpa.

At least 289 records · Page 16Linked to original sources
PubMed1973

Mechanisms for intracellular calcium regulation in heart. I. Stopped-flow measurements of Ca++ uptake by cardiac mitochondria.

Initial velocities of energy-dependent Ca(++) uptake were measured by stopped-flow and dual-wavelength techniques in mitochondria isolated from hearts of rats, guinea pigs, squirrels, pigeons, and frogs. The rate of Ca(++) uptake by rat heart mitochondria was 0.05 nmol/mg/s at 5 microM Ca(++) and increased sigmoidally to 8 nmol/mg/s at 200 microM Ca(++). A Hill plot of the data yields a straight line with slope n of 2, indicating a cooperativity for Ca(++) transport in cardiac mitochondria. Comparable rates of Ca(++) uptake and sigmoidal plots were obtained with mitochondria from other mammalian hearts. On the other hand, the rates of Ca(++) uptake by frog heart mitochondria were higher at any Ca(++) concentrations. The half-maximal rate of Ca(++) transport was observed at 30, 60, 72, 87, 92 microM Ca(++) for cardiac mitochondria from frog, squirrel, pigeon, guinea pig, and rat, respectively. The sigmoidicity and the high apparent K(m) render mitochondrial Ca(++) uptake slow below 10 microM. At these concentrations the rate of Ca(++) uptake by cardiac mitochondria in vitro and the amount of mitochondria present in the heart are not consistent with the amount of Ca(++) to be sequestered in vivo during heart relaxation. Therefore, it appears that, at least in mammalian hearts, the energy-linked transport of Ca(++) by mitochondria is inadequate for regulating the beat-to-beat Ca(++) cycle. The results obtained and the proposed cooperativity for mitochondrial Ca(++) uptake are discussed in terms of physiological regulation of intracellular Ca(++) homeostasis in cardiac cells.

Animals↗
PubMed1972

The effect of calcium ionophores on fragmented sarcoplasmic reticulum.

X-537 A and A 23187, two antibiotics which form liphophilic complexes with divalent cations, function as ionophores in vesicular fragments of sarcoplasmic reticulum (SR). Addition of either ionophore to SR preloaded with calcium in the presence of adenosine triphosphate (ATP), causes rapid release of calcium. Furthermore, net calcium accumulation by SR is prevented, when the ionophores are added to the reaction mixture before ATP. On the contrary, ATP-independent calcium binding to SR is not inhibited. This effect is specific for the two antibiotics and could not be reproduced, either by inactive derivatives, or by other known ionophores. Neither ionophore produces alterations of the electron microscopic appearance of SR membranes or inhibition of the calcium-dependent ATPase. In fact, the burst of ATP hydrolysis obtained on addition of calcium, is prolonged in the presence of the ionophores. Lanthanum inhibits ATP-independent calcium binding to SR, ATP-dependent calcium accumulation and calcium-dependent ATPase. However, addition of lanthanum to SR preloaded in the presence of ATP, does not cause calcium release. The reported experiments indicated that: (a) ATP-dependent calcium accumulation by SR results in primary formation of calcium ion gradients across the membrane. (b) Most of the accumulated calcium is not available for displacement by lanthanum on the outer surface of the membrane. (c) Calcium ionophores induce rapid equilibration of the gradients, by facilitating cation diffusion across the membrane.

Adenosine Triphosphatases↗