Magnesium net fluxes and distribution in rabbit myocardium in irreversible contracture.

Distribution of magnesium (Mg) in heart muscle was studied by measuring fluxes of Mg and transmembrane potentials as a function of perfusate [Mg2+] after a massive increase in permeability of the sarcolemma was induced in the Langendorff prepared heart from the Nembutal-anesthetized rabbit. After onset of 0 mM [Ca2+] perfusion which produced excitation-contraction (E-C) uncoupling and mechanical arrest, action potentials recorded from subepicardial cells showed an increase in duration and decrease in amplitude, which progressed until no transmembrane potentials could be observed. Restoration of physiological salt solution perfusion after 15 min of [Ca2+]-free perfusion caused an irreversible contracture that was associated with 1) efflux of potassium (K) and myoglobin, 2) perfusate [Mg2+]-dependent flux of Mg, and 3) transmembrane potentials of 0 mV. The magnitude of net efflux of K and myoglobin during contracture was unaffected by perfusate [Mg2+]. During the first 2 min of contracture, net efflux of Mg (mumoles per gram wet muscle +/- SE) was 1.37 +/- 0.09 and 0.48 +/- 0.19 during 0 mM and 2.5 mM [Mg2+] perfusion, respectively; but a net influx of 0.56 +/- 0.23 occurred during 5 mM [Mg2+] perfusion. Total sarcoplasmic [Mg] may correspond to perfusate [Mg2+] of 3.6 mM, which was found by interpolation to prevent any net flux of Mg during contracture. 3.6 mM may, therefore, represent the upper limit of the intracellular free-ionized Mg concentration in rabbit heart.

Tonometric measurement of dental pulpal and mandibular marrow blood pressures.

Dental pulpal and mandibular marrow vascular pressures were studied with a tonometric method under conditions of autoperfusion and step flow controlled perfusion. The tonometric pulpal and marrow pressures averaged 17 +/- 5 mm Hg and 20 +/- 6 mm Hg, respectively. The venous end pressure was 16 +/- 8 mm Hg at a perfusion pressure of 156 +/- 28 mm Hg. The tonometric pulpal pressures were lower and were less variable than the comparable canulation pulpal pressures.

Analysis of ion distribution in subcellular myocardial particulates separated by density gradient centrifugation with nonpolar liquid under ion immobilization conditions.

Methods have been developed for the measurement of the Ca and Mg contents of subcellular particulates, separated by nonpolar liquid density gradient ultracentrifugation. After rapid deep freezing and removal of water by freeze-drying the frozen crushed tissue, homogenization and sonification a nonpolar liquid were carried out. Application of this technique has yielded evidence that the relative amounts of Mg and Ca are greatly different in the several subcellular particulates of normal heart muscle separated by density gradient centrifugation. It also appears that the comparative specific activity of Ca, after loading the perfusates with 45Ca, is very different in the several density fractions of the normal heart. The density site of greatest loading with 45Ca is changed in Ca2+-lack arrest and is very greatly altered by the induction of paradoxical Ca contracture. The method of ion-immobilization separation for water-free particulates of different densities in nonpolar liquids appears to be applicable to analysis for various constituents of other biological materials.

K+ and Mg-2+ net fluxes in relation to zero [Ca-2+] perfusion and subsequent cardiac contracture.

Following within 45 sec after the development of contracture induced by restoring normal ionic composition perfusion conditions after a 12 min period of mechanical arrest in the rabbit heart caused by zero [Ca-2+] perfusion, there is an explosive efflux of K+ and Mg-2+. After shorter periods of Ca-2+-lack arrest, the restoration of [Ca-2+] to normal causes recovery of rhythmic contraction and no K+ efflux. The K+ and Mg-2+ effuxes are ascribed to the effects of the contracture itself and not simply to the loss of Ca-2+ during zero [Ca-2+] arrest nor to the restoration of normal perfusate [Ca-2+], except insofar as the latter operates to induce the contracture. It is suggested that cell membrane permeability progressively increases during zero [Ca-2+] arrest and that an abnormally large influx of Ca-2+ after restoration of normal perfusate [Ca-2+] induces the contracture.

Localization of rapidly exchangeable calcium in mammalian heart muscle.

Two new methods were used to calculate net labeled Ca++ compartmental influx into rabbit myocardium: 1) continuous measurements of arteriovenous differences in 45Ca levels are corrected for longitudinal mixing and for varying transit times by a double-isotope technique with the relatively impermeant 131I-labeled albumin as the referent substance; 21 tritiated sucrose and 45Ca contents were measured in ventricular muscle after perfusion with the doubly labeled perfusate and washout of aqueous perfusate with fluorocarbon 80. Both methods yielded similar results. Only 52% of the Ca++ efflux from the vascular bed during the rapid flux period can be accounted for as Ca++ in interstitial water under control perfusion conditions. Contracture induced by substituting K+ for Na+ in the perfusate brings the fraction not accounted for as Ca++ in interstitial water from 48 to 60% of the total in 5 min. When irreversible contracture is induced by return to normal [Ca++] after short periods of excitation-contraction uncoupling due to zero [Ca++] perfusion, the fraction not accounted for as Ca++ in interstitial water is 68% in 5 min.

Some physiological aspects of bone marrow pressure.

An investigation was made into some of the hemodynamic properties of bone. The parameters studied were bone marrow and medullary venous pressures. The resting level and changes in the two pressures were measured under autoperfusion and constant-flow perfusion conditions. Changes were induced either by drugs or by alteration in blood flow. Marrow pressure was measured with a new tonometric technique. A specially constructed miniature pressure transducer was utilized as the tonometer. Trauma-induced hemorrhage of the marrow was avoided. The marrow pressure was measured across an intact endosteal membrane. The pressure of the normal bone marrow and the medullary venous pressure in the anesthetized dog were nearly equal, approximately twenty millimeters of mercury and about 17 per cent of systemic blood pressure. These values were found whether measured simultaneously or separately in comparable groups of dogs under a variety of conditions. Induced changes in the two pressures produced either by changes in perfusion rate or by drugs were also nearly equal and in the same direction.