Effects of angiotensin, catecholamines and cyclic AMP on calcium storage in aortic microsomes.
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1. The binding of calcium observed in microsomal membranes derived from the intimal-medial layer of rabbit aorta was specifically increased by the magnesium salt of adenosine triphosphate (Mg-ATP). This calcium uptake was inhibited by potassium and sodium.2. Angiotensin II, both in the presence and in the absence of Mg-ATP, reduced the binding of calcium and increased the release of membrane-incorporated calcium. These effects were dose-dependent. Analogues of angiotensin II devoid of intrinsic activity, failed to alter the release of calcium.3. Dibutyryl-cyclic AMP increased the binding of calcium and reduced the rate of its release. The maximal effect was observed at the concentration of 10(-5)M.4. (-)-Noradrenaline reduced the binding of calcium; this effect was inhibited by phenoxybenzamine. Conversely, adrenaline like cyclic AMP, increased the calcium binding; the effect of adrenaline was suppressed by a beta-adrenoceptor blocking agent.5. These observations demonstrate the existence of membranes in rabbit aorta capable of storing calcium. Excitatory drugs seem to affect directly and specifically the binding and release of calcium in these membranes. (+info)
Effect of phospholipase C on calcium release from epithelia treated with antidiuretic hormone.
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1. Addition of antidiuretic hormone (ADH) to the bladders of toads (Bufo marinus) preloaded with (45)Ca causes an increase in the rate of calcium efflux.2. Pretreatment of the serosal surfaces of bladders with phospholipase C prevents the action of ADH on calcium efflux. (+info)
Embryonic chick intestine in organ culture. A unique system for the study of the intestinal calcium absorptive mechanism.
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Duodena from 20-day-old chick embryos can be maintained in large scale organ culture on specially designed stainless-steel grids in contact with serum-free medium for 48 h with excellent preservation of mucosal structure at both the light and electron microscope levels. Although mitotic rate was subnormal, several other factors attest to the essential viability of the cultured intestine: L-leucine incorporation into protein, as well as the synthesis of a specific vitamin D(3)-induced calcium-binding protein (CaBP), increased over a 48-h culture period, and the electropotential gradient across the intestine was maintained throughout the culture period as was a concentration gradient for calcium. The tissue responded to vitamin D(3) in the medium by synthesizing the calcium-binding protein within 6 h and by exhibiting enhanced (45)Ca uptake within 12-24 h. Concentrations of vitamin D(3), or its 25-hydroxylated derivative, higher than necessary for CaBP induction, also increased the activity of alkaline phosphatase. The 1,25-dihydroxylated derivative of vitamin D(3), at a level extremely potent in CaBP induction, did not stimulate alkaline phosphatase. Mucosal to serosal transport of (45)Ca could also be measured in everted duodenal sacs, subsequent to culture under similar conditions, and was also increased by vitamin D(3) in the medium. Other embryonic organs, esophagus, stomach, liver, pancreas, lung, skin, and muscle, did not produce CaBP in response to vitamin D(3) in the culture medium. However, CaBP-synthesizing capacity was present in the entire intestinal tract, exclusive of the rectum. (59)Fe and (32)P uptake by cultured duodenum were also stimulated by vitamin D(3). The system has proven quite useful in the study of the vitamin D-mediated calcium absorptive mechanism but should be applicable to the study of the absorption of other nutrients, drugs, hormones, etc., as well as other studies of intestinal function. (+info)
Biological activity of 1alpha-hydroxycholecalciferol, a synthetic analog of the hormonal form of vitamin D3.
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1,25-Dihydroxycholecalciferol, the apparent active hormonal form of cholecalciferol (vitamin D(2)), is formed from cholecalciferol by specific and sequential hydroxylations of the sterol at carbons 25 and 1. Recently, 1alpha-hydroxycholecalciferol was synthesized and we report on its biological activity in rachitic chicks. 1alpha-Hydroxycholecalciferol is identical in potency to 1,25-dihydroxycholecalciferol in stimulation of intestinal calcium absorption; either sterol elicits a near maximal effect at a dose of 0.3-0.6 nmol. The time-course of action of 1alpha-hydroxycholecalciferol also parallels that of the active metabolite 1,25-dihydroxycholecalciferol with a maximal increase in calcium transport occurring 5-10 hr after administration of sterol to vitamin D-deficient chicks. 6.5 nmol of 1alpha-hydroxycholecalciferol causes a doubling in calcium absorption in only 2-3 hr, which is the most rapid physiologic response yet detected for a vitamin D-sterol. 1alpha-Hydroxycholecalciferol is active also in enhancing bone calcium resorption and, like 1,25-dihydroxycholecalciferol, is at least 10 times as active as cholecalciferol in mobilizing bone calcium and raising plasma calcium concentration. It is concluded that 1alpha-hydroxycholecalciferol represents a synthetic analog of 1,25-dihydroxycholecalciferol that can be used both to study the mechanism of action of this hormone and as a therapeutic agent in the treatment of patients with certain metabolic bone diseases. (+info)
Calcium ions and the cardiotonic action of glucagon (calcium fluxes-myocardial function-asystole-membrane transport-cat).
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The positive inotropic action of glucagon on the cat heart is dependent upon the extracellular Ca concentration. The inotropic effect is proportionately greater, the lower the concentration of Ca in the perfusate (above zero, at which glucagon is without significant effect). Nevertheless, Ca influx (as measured with (45)Ca) into the myocardium is not demonstrably influenced by glucagon, except at 0.09 mM Ca, where the interpretation of the effect is complicated by the fact that there is mechanical asystole at that Ca concentration in the absence of glucagon, while excitation-contraction coupling still occurs when the hormone is added. The possibility is suggested that glucagon may influence intracellular movement of Ca, even when its transmembrane movement is affected only slightly. (+info)
Calcium-like action of phenethylbiguanide and related compounds: inhibition of pyruvate kinase.
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Pyruvate kinase (EC 2.7.1.40) is inhibited by phenethylbiguanide. The kinetics of inhibition are competitive between biguanide and divalent, but not monovalent, metal cation activators of the enzyme; biguanide inhibition thus resembles inhibition by Ca(++). Alteration of either the polar or nonpolar portion of the phenethylbiguanide molecule quantitatively reduces its effectiveness as an inhibitor of pyruvate kinase, but the kinetics of inhibition remain qualitatively unchanged. Measurements of [(3)H]phenethylbiguanide binding to the enzyme indicate the presence of a single class of about 12 binding sites per enzyme molecule; binding characteristics are not significantly different in the presence of either monovalent or divalent metal cations. Studies with (45)Ca(++) and (54)Mn(++) demonstrate about 4 metal binding sites per enzyme molecule; phenethylbiguanide displaces these metal cations from the enzyme. Studies with several enzymes, dependent upon divalent metal cations, of both metal-bridge and substrate-bridge classes fail to show significant inhibition except at much higher phenethylbiguanide concentrations. (+info)
Calcium-dependent norepinephrine release from presynaptic nerve endings in vitro.
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Potassium concentrations above 15-20 mM in the medium, or addition of 75 muM veratridine to medium that contains 5 mM K, stimulate calcium accumulation and Ca-dependent norepinephrine release by presynaptic nerve terminals (synaptosomes) incubated in vitro. The effects of veratridine, but not of K, are blocked by 0.2 muM tetrodotoxin. This association between Ca entry and norepinephrine release is consistent with the Calcium Hypothesis of transmitter release. These results add further evidence that synaptosomes may retain many functional properties of intact presynaptic endings. (+info)
Effects of colicins E1 and K on permeability to magnesium and cobaltous ions.
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The energy-dependent exchange of intracellular Mg(2+) with extracellular Mg(2+) or Co(2+) is inhibited by colicin E1 and, less strongly, by colicin K. Treatment with either colicin causes a net loss of intracellular Mg(2+). This loss begins immediately in cells treated with colicin E1, but in colicin K-treated cells the onset of Mg(2+) loss is delayed 1 to 10 min, depending upon the temperature and the multiplicity of colicin K. Both colicins differ from chemical inhibitors of energy-yielding metabolism; energy poisons block transport of Mg(2+) and Co(2+), but both colicins increase passive permeability to Mg(2+) and Co(2+). Inhibitors of energy-yielding metabolism (and of Mg(2+) exchange) block the initiation of Mg(2+) loss by either colicin, but do not stop colicin-promoted efflux once it has begun. Colicin E1 added before colicin K prevents the more rapid Mg(2+) efflux characteristic of colicin K-treated cells. Quantitative comparisons of the effects of colicins E1 and K upon permeability to Mg(2+) and Co(2+) lead us to conclude that the two colicins are not identical in their mode of action. (+info)