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G McClellan

Publications and source records attributed to G McClellan.

At least 37 records · Page 2Linked to original sources
PubMed1987

Ca-independent regulation of cardiac myosin.

Calcium-independent regulation of the contractile proteins of cardiac muscle has been studied using hyperpermeable cells from rat ventricles and sections of quickly-frozen rat hearts. These preparations have been used to study maximum Ca-activated force, myosin ATPase activity and the maximum velocity of unloaded shortening. Beta adrenergic activity increases the amount of force and the ATPase activity in accordance with the concentration of the V1 isozyme of myosin. V3 activity is decreased at the same time. In tissues containing only V1, there is no change in maximum velocity in response to beta adrenergic stimulation. These results indicate that beta adrenergic stimulation recruits V1 force generators and probably regulates the transition between a Ca unresponsive and a Ca responsive force generator. This type of regulation provides the cell with the ability to operate along many different force-velocity relations.

Adenosine Triphosphatases↗
PubMed1987

Absence of myelin basic protein in granular cell tumors.

Thirty-one granular cell tumors were studied for the presence of myelin basic protein (MBP) using a monoclonal antibody with a characterized specificity and an immunoperoxidase technique. The tumors studied were from all areas of the body, from both sexes, and from nine decades of age. All the tumors stained negatively for MBP. One brief report using a monoclonal antibody had noted positive staining for MBP in granular cell tumors. A more recent study using polyclonal antibodies found negative staining for MBP in Schwann cell neoplasms including granular cell tumors. This study, using an immunohistochemical technique, demonstrated the absence of an MBP marker in granular cell tumors of diverse location despite the fact that granular cell tumors are widely held to be of Schwann cell origin.

Antibodies, Monoclonal↗
PubMed1987

Beta-adrenergic regulation of cardiac myosin.

Calcium-independent regulation of the contractile proteins of cardiac muscle has been studied using hyperpermeable cells from rat ventricles and sections of quickly frozen rat hearts. These preparations have been used to study maximum calcium-activated force, myosin ATPase activity, and the maximum velocity of unloaded shortening. Beta-adrenergic activity increases the amount of force and the ATPase activity in accordance with the concentration of the V1 isozyme of myosin. V3 activity is decreased at the same time. In tissues containing only V1, there is no change in maximum velocity in response to beta-adrenergic stimulation. These results indicate that beta-adrenergic stimulation recruits V1 force generators and probably regulates a transition between a calcium unresponsive and a calcium responsive force generator.

Actins↗
PubMed1986

Adrenergic regulation of myosin adenosine triphosphatase activity.

The amount of inorganic phosphate liberated by the adenosine triphosphatase activity of myosin in a thin section of cardiac tissue can be measured quantitatively by precipitation with calcium in an alkaline medium under a defined set of conditions. Specificity of the procedure for myosin adenosine triphosphatase has been confirmed by the response to inhibitors and to different degrees of contractile filament overlap. Precise quantitation of adenosine triphosphatase activity has been demonstrated by (1) constant rate over time, (2) linearity with amount of enzyme, (3) correct values for the Km of adenosine triphosphate, and (4) a similar value for Vmax to those determined by more traditional procedures. Stimulation of the beta-adrenergic system by the release of catecholamines following injection of the animal with 6-hydroxydopamine causes a rise and then a fall of both calcium- and actin-activated adenosine triphosphatase in parallel with the changes in blood levels of the transmitter. Tyramine injection of rats produces a dose related increase in myosin adenosine triphosphatase. Perfusion of isolated hearts with isoproterenol increases myosin adenosine triphosphatase in dose-related manner. Addition of cyclic adenosine monophosphate and phosphodiesterase inhibitor to the solution bathing frozen, dried sections of heart increases both calcium- and actin-activated adenosine triphosphatase activity by almost 150%. The data show that the beta-adrenergic system, through cyclic adenosine monophosphatate, regulates the enzymatic activity of myosin, independent of the concentration of calcium. The possible role of this regulatory mechanism in the physiological modulation of cardiac contractility is discussed.

Actins↗
PubMed1986

Calcium-sensitive cellular and subcellular transport of sodium, potassium, magnesium, and calcium in sodium-loaded vascular smooth muscle. Electron probe analysis.

Electron probe x-ray microanalysis of the composition of rabbit portal anterior mesenteric vein smooth muscle was performed following sodium loading and washout into sodium-free lithium solutions. Sodium and lithium were also measured with atomic absorption spectrophotometry. Cellular uptake of sodium and loss of potassium during sodium loading were much faster at high (37 degrees C) than at low (2 degrees C) temperature, as was the passive ouabain-resistant uptake of potassium during lithium washout. The loss of sodium at 2 degrees C into lithium solution consisted of two components: a rapid efflux that was complete by 30 minutes, and a slow component that required at least 24 hours for completion. The amount of sodium lost through the first component (approximately 200-300 mmol/kg dry weight) was relatively independent of the amount of sodium loading. The loss of cellular sodium at 2 degrees C, after 30 minutes, was accompanied by a gain of cellular lithium. Ouabain-resistant sodium loss and lithium and potassium uptake were markedly accelerated at 37 degrees C; sodium loss was complete (1200 mmol sodium/kg dry weight lost) by 30 minutes of washout. Sodium-loaded cells also lost chloride ion and gained magnesium during sodium efflux at 37 degrees C. Mitochondrial and nuclear sodium and potassium were correlated with the respective cytoplasmic concentrations during both sodium loading and sodium washout, indicating the relatively rapid equilibration of the monovalent ions between the cytoplasm and organelles. Calcium-free solutions markedly inhibited the ouabain-resistant sodium and chloride ion effluxes and potassium influx in muscles incubated, after sodium loading, in lithium solutions at 37 degrees C. These fluxes could be restored to near normal values by 0.2 mM calcium. The calcium sensitivity of the ouabain-resistant sodium, potassium, and chloride ion fluxes observed in this and other studies raises the possibility that some abnormalities of monovalent ion transport observed in cells of hypertensives are secondary to changes in cellular calcium.

Animals↗
PubMed1986

The properties of cardiac contractile proteins are modulated by autonomic innervation.

The force generators in myocardial cells of rats and rabbits can exist in three different states: relaxed and calcium unresponsive; relaxed and calcium responsive; and contracted. The transition between the two calcium responsive states is produced by the abrupt rise in the concentration of calcium ions during activation by depolarization of the surface membrane. The transition between the two relaxed states is controlled by the beta-adrenergic system. Stimulation converts calcium unresponsive to calcium responsive force generators by causing the release of a 21,000 dalton regulatory factor from intracellular sites on a membrane. The factor then interacts with myosin. The regulatory system can distinguish between V1 and V3 myosins, producing a calcium responsive state in the former and a calcium unresponsive state in the latter. The result of beta-adrenergic activity is, therefore, an increase in the faster and a decrease in the slower force generators. As a result individual cardiac cells can have many different force-velocity relations.

Animals↗
PubMed1985

Electron probe X-ray microanalysis of post-tetanic Ca2+ and Mg2+ movements across the sarcoplasmic reticulum in situ.

Ca2+ and Mg2+ movements across the sarcoplasmic reticulum (SR) of frog skeletal muscle fibers were measured in situ by electron probe microanalysis of muscles rapidly frozen following a tetanus. At 400 ms following a 1.2-s tetanus at room temperature, the force had relaxed to base-line, and 0.3 mmol of Ca2+/liter of cytoplasmic H2O had been pumped by the SR, indicating that the in situ pumping of the SR Ca-ATPase is sufficiently high to account for the removal of Ca2+ from the Ca2+-specific sites of troponin (0.18 mmol of Ca2+-specific sites/liter of cytoplasmic H2O) and for the rate of relaxation from a tetanus at room temperature. The half-time of the return of the total 1.0 mmol of Ca2+/liter of cytoplasmic H2O released during a tetanus was 1.1 s, comparable to the slow Koff rate of Ca2+ from (carp) parvalbumin (1.0 s-1) and consistent with the hypothesis that the return of this Ca2+ to the terminal cisternae is rate-limited by the Ca2+ off-rate from parvalbumin. The return of the Mg2+ taken up by the terminal cisternae during a tetanus to resting levels was significantly slower than the time course of the Ca2+ movements, suggesting that the Mg2+ permeability of the SR in situ is low and may be transiently increased during tetanic stimulation.

Animals↗
PubMed1984

Comparative histological and radiographic effects of CO2 laser versus standard surgical anterior cervical discectomy in the dog.

The use of the CO2 laser in the extirpation of selected intracranial and spinal tumors has been shown to be of great benefit. The role of the CO2 laser as applied to other neurosurgical procedures has not been adequately explored or defined. The authors have compared the radiological and histological effects of performing anterior cervical discectomy with the CO2 laser to those of performing it by standard operative technique. Groups of laboratory dogs underwent anterior cervical discectomy performed in both operative fashions. The animals were killed and the operated disc spaces were examined radiographically and microscopically. Observations were made 1, 3, 5, 8, and 10 weeks postoperatively. Based upon these radiographic and microscopic evaluations, the authors were able to draw certain conclusions with respect to the use of the CO2 laser in performing anterior cervical discectomy.

Animals↗
PubMed1983

Regulation of calcium sensitivity in perforated mammalian cardiac cells.

Sarcolemmal perforations can be produced in bundles of rat right ventricular cells by either perfusion of the heart or soaking of the bundles with a solution containing 10 mM EGTA. All cells are affected and lose approximately 40% of the surface membrane. In these cells it is possible to show cAMP regulation of contractility (maximum Ca-activated force) without cAMP regulation of Ca sensitivity (pCa for 50% of maximum Ca-activated force). Therefore, the target molecule for cAMP is different for the two regulatory systems. Both regulatory systems can be slowly washed out of the cell by 10 mM EGTA solution but not by relaxing or contraction solutions. A model for regulation of Ca sensitivity is proposed.

Animals↗
PubMed1983

Cyclic AMP regulation of myosin isozymes in mammalian cardiac muscle.

Hyperpermeable cells from rat heart contain a cAMP-dependent system that can increase the maximum Ca-activated force (contractility) of the contractile proteins. In two different conditions where the relative concentration of the myosin isozymes changes, i.e., hypothyroidism and aging, the size of the increase in contractility from activation of the cAMP-regulated system varies closely with the relative concentration of V1, the isozyme of myosin with the greatest Ca- and actin-activated ATPase activity. The existence of another system for the regulation of the slow isozyme V3 has been demonstrated, and it may be inhibited by beta-adrenergic activity. The possibility of cAMP-dependent myosin regulation of contraction in addition to Ca regulation of troponin is considered. Phosphorylation of the contractile proteins themselves is not required for the increased contractility.

Animals↗
PubMed1983

Energy transport from mitochondria to myofibril by a creatine phosphate shuttle in cardiac cells.

In hyperpermeable cardiac cells, in which the surface membrane has been made highly permeable to small molecules and ions, resting tension increases when the concentration of ATP falls below 200 microM. Peak resting tension occurs in 10 microM ATP and equals 60% of maximum Ca-activated force in 5 mM ATP. The mitochondria in hyperpermeable cells can maintain an ATP concentration above 200 microM if supplied with O2, substrate, ADP, and inorganic phosphate (Pi). Removal of ATP from the bathing solution does not increase resting tension as long as creatine phosphate is present. However O2, substrate, and Pi cannot lower resting tension in the absence of ATP and creatine phosphate. These results are interpreted as evidence for adenine nucleotide tightly bound to the myofibrils and a creatine phosphate shunt of energy from the mitochondria to the myofibrils.

Adenosine Diphosphate↗
PubMed1983

Regulation of cardiac contractile proteins by phosphorylation.

Several of the contractile proteins of the heart can be phosphorylated, but in studies with isolated proteins only phosphorylation of the inhibitory subunit of troponin (TnI) produces a major change in the properties of the contractile system. As TnI is phosphorylated, the concentration of calcium required for activation of contraction is increased. Phosphorylation of the tropomyosin-binding subunit of troponin (TnT) or of the light chain of myosin fails to change ATPase activity of the isolated protein system. Phosphorylation of TnI is stimulated by the beta-adrenergic system and inhibited by the cholinergic system. Maximum calcium-activated force produced by the contractile system can be increased in hyperpermeable cardiac cells by cyclic AmP (cAMP) or agents that stimulate cAMP synthesis. This change in the contractile system, which appears to be part of the physiological response to beta-adrenergic stimulation, is mediated by phosphorylation of an intermediate that then modifies the contractile system. Phosphorylation of the contractile proteins is not involved.

Adenosine Triphosphate↗
PubMed1982

Histochemical detection of specific isozymes of myosin in rat ventricular cells.

A histochemical method for distinguishing isozymes of myosin in rat ventricles has been developed. The procedure involves preincubation in pH 10.5, which inhibits Ca-activated ATPase of the V3 isozyme but not the V1 isozyme of myosin. The specificity of the technique has been demonstrated by comparison of results in hearts from young euthyroid and hypothyroid rats, in which the predominant isozymes are, respectively, V1 and V3. The technique is capable of detecting as small a change in the relative amount of V1 as 15% of the total myosin. Isoenzymes appear to be uniformly distributed within each ventricular cell. There is only a small difference in the content of V1 among the cells in a ventricular chamber of hearts from young euthyroid and hypothyroid rats, but in the period of rapid transition of isozyme content after thyroidectomy, there is considerable heterogeneity of V1 concentration among the cells. The functional implications of the mixture of isozymes is discussed.

Adenosine Triphosphatases↗
PubMed1981

Morphometric analysis of secretory granules and prolactin levels in chromophobe pituitary adenoma.

Morphometric analysis of secretory granules was performed in 16 chromophobe adenomas: seven undifferentiated cell adenomas (group A), two of which were accompanied by mild hyperprolactinemia and nine prolactin cell sparsely granulated adenomas (group B), all associated with hyperprolactinemia. The results of morphometry were correlated with levels of prolactin in tumor and serum. The range of granule diameters was similar in both groups but granule size frequency was different resulting in a characteristic histogram for each group. Poor relationship existed between granule mean diameter and prolactin in tissue of the group B. In the same group there was virtually no linear relationship between serum prolactin on one side and granule mean diameter, granule density or tissue prolactin on the other. Also in the group B a paradoxical inverse relationship was found between granule density and tumor prolactin. Apparently, in prolactin cell adenoma a sizable part of hormone is located outside of mature secretory granules. An abundance of immature granules, misplaced exocytosis, crinophagia or defect in packaging of the hormone could all explain this paradox but the last possibility seems most plausible.

Adenoma, Chromophobe↗
PubMed1981

Calcium release and ionic changes in the sarcoplasmic reticulum of tetanized muscle: an electron-probe study.

Approximately 60-70% of the total fiber calcium was localized in the terminal cisternae (TC) in resting frog muscle as determined by electron-probe analysis of ultrathin cryosections. During a 1.2 s tetanus, 59% (69 mmol/kg dry TC) of the calcium content of the TC was released, enough to raise total cytoplasmic calcium concentration by approximately 1 mM. This is equivalent to the concentration of binding sites on the calcium-binding proteins (troponin and parvalbumin) in frog muscle. Calcium release was associated with a significant uptake of magnesium and potassium into the TC, but the amount of calcium released exceeded the total measured cation accumulation by 62 mEq/kg dry weight. It is suggested that most of the charge deficit is apparent, and charge compensation is achieved by movement of protons into the sarcoplasmic reticulum (SR) and/or by the movement of organic co- or counterions not measured by energy dispersive electron-probe analysis. There was no significant change in the sodium or chlorine content of the TC during tetanus. The unchanged distribution of a permeant anion, chloride, argues against the existence of a large and sustained transSR potential during tetanus, if the chloride permeability of the in situ SR is as high as suggested by measurements on fractionated SR. The calcium content of the longitudinal SR (LSR) during tetanus did not show the LSR to be a major site of calcium storage and delayed return to the TC. The potassium concentration in the LSR was not significantly different from the adjacent cytoplasmic concentration. Analysis of small areas of I-band and large areas, including several sarcomeres, suggested that chloride is anisotropically distributed, with some of it probably bound to myosin. In contrast, the distribution of potassium in the fiber cytoplasm followed the water distribution. The mitochondrial concentration of calcium was low and did not change significantly during a tetanus. The TC of both tetanized and resting freeze-substituted muscles contained electron-lucent circular areas. The appearance of the TC showed no evidence of major volume changes during tetanus, in agreement with the estimates of unchanged (approximately 72%) water content of the TC obtained with electron-probe analysis.

Animals↗
PubMed1978

Composition of vacuoles and sarcoplasmic reticulum in fatigued muscle: electron probe analysis.

Electron probe analysis, cryo-ultramicrotomy, and freeze-substitution were used to determine the nature of vacuolation and the subcellular composition in fatigued frog skeletal muscle fibers. The vacuoles caused by fatigue were part of the T-tubule system and contained high concentrations of NaCl. The calcium concentration in the terminal cisternae was higher than previously measured normal resting values. Mitochondrial calcium content was relatively low (mean +/- SEM, 2 +/- 2 mmol/kg dry weight). Fiber NaCl was increased. It is concluded that fatigue is not due to the depletion of calcium stores from the terminal cisternae or to uncoupling of mitochondria due to calcium loading but may be caused by multiple mechanisms including failure of the T-tubule action potential.

Animals↗