Glucose as a lipolytic agent: studies on isolated rat adipocytes

In order to elucidate the direct effect of glucose on lipolysis in isolated rat adipocytes, cells were incubated in a buffer with different concentrations of this sugar: 2, 8 or 16 mmol/l. The increase in glucose concentration from 2 mmol/l to 8 or 16 mmol/l enhanced basal lipolysis by 30% and 47%, respectively. Epinephrine-induced lipolysis (1 micromol/l) was also increased by 31% and 32%, when glucose concentration was increased from 2 mmol/l to 8 or 16 mmol/l, respectively. The rise in lipolysis caused by glucose was restricted by H-89 (an inhibitor of protein kinase A, 30 micromol/l), but insulin (1 nmol/l) had no inhibitory action. The augmentation of lipolysis by glucose did not require its metabolism (as demonstrated using 2-deoxyglucose) and was due to the action of this sugar on the final steps of the lipolytic cascade, particularly on protein kinase A. However, short-term exposure of adipocytes to higher glucose concentrations did not restrict the inhibitory action of insulin on lipolysis induced by epinephrine.     

Insulin as a surface marker on isolated cells from rat pancreatic islets

Immunoreactive insulin was shown to exist as a surface molecule in the plasma membrane of dispersed rat pancreatic islet cells. The intact cells were stained by immunofluorescence with a guinea pig antisera specific for insulin. The hormone on the cell surface could not be accounted for by insulin bound to specific receptors or nonspecifically absorbed to cells. Thus, surface insulin was demonstrated to be a specific membrane antigen for islet cells. Furthermore, the proportion of islet cells with insulin on the cell surface was directly correlated with insulin secretion in several different settings. This correspondence was demonstrated by varying the glucose concentration in the medium, by withholding Ca2+, which inhibits secretion, and by adding theophylline, which potentiates secretion. Consequently, these results suggested that insulin as a membrane protein was a marker for cells that actively secreted the hormone and may have been derived in the fusion process of secretory granules with the plasma membrane.     

Impact of detubulation on force and kinetics of cardiac muscle contraction

Action potential–driven Ca²⁺ currents from the transverse tubules (t-tubules) trigger synchronous Ca²⁺ release from the sarcoplasmic reticulum of cardiomyocytes. Loss of t-tubules has been reported in cardiac diseases, including heart failure, but the effect of uncoupling t-tubules from the sarcolemma on cardiac muscle mechanics remains largely unknown. We dissected intact rat right ventricular trabeculae and compared force, sarcomere length, and intracellular Ca²⁺ in control trabeculae with trabeculae in which the t-tubules were uncoupled from the plasma membrane by formamide-induced osmotic shock (detubulation). We verified disconnection of a consistent fraction of t-tubules from the sarcolemma by two-photon fluorescence imaging of FM4-64–labeled membranes and by the absence of tubular action potential, which was recorded by random access multiphoton microscopy in combination with a voltage-sensitive dye (Di-4-AN(F)EPPTEA). Detubulation reduced the amplitude and prolonged the duration of Ca²⁺ transients, leading to slower kinetics of force generation and relaxation and reduced twitch tension (1 Hz, 30°C, 1.5 mM [Ca²⁺]_o). No mechanical changes were observed in rat left atrial trabeculae after formamide shock, consistent with the lack of t-tubules in rodent atrial myocytes. Detubulation diminished the rate-dependent increase of Ca²⁺-transient amplitude and twitch force. However, maximal twitch tension at high [Ca²⁺]_o or in post-rest potentiated beats was unaffected, although contraction kinetics were slower. The ryanodine receptor (RyR)2 Ca-sensitizing agent caffeine (200 µM), which increases the velocity of transverse Ca²⁺ release propagation in detubulated cardiomyocytes, rescued the depressed contractile force and the slower twitch kinetics of detubulated trabeculae, with negligible effects in controls. We conclude that partial loss of t-tubules leads to myocardial contractile abnormalities that can be rescued by enhancing and accelerating the propagation of Ca²⁺-induced Ca²⁺ release to orphan RyR2 clusters.     

Frequency-dependent blockade of sodium channels in isolated rat myocardial cells by the anti-arrhythmia agent ethmozine

A patch-clamp method was used to study the effects of the phenotiazine antiarrhythmic drug ethmozine (E) on the fast sodium inward current (INa) in freshly isolated heart muscle cells of adult rats. At a concentration of 10(-5) M E caused INa inhibition that could be enhanced by increasing the frequency of depolarization. This inhibition was reversible. After the termination of repetitive depolarization the amplitude of INa recovered with a time constant of about 10 sec. These findings may help to explain the therapeutic efficiency of E in high frequency cardiac rhythm disturbances.     

Validation of HPLC analysis method of a novel antihypertensive agent MS23 in rat plasma

MS23 is a vasodilator with unique dual action pharmacological profile to inhibit type 4 PDE and antagonize L-type calcium channels. We validated an analytical protocol for MS23 in rat plasma using high performance liquid chromatography (HPLC). A C18 column and a phosphate/acetonitrite buffer were used for chromatographic separation. UV detection was performed at 307 nm. The calibration curve for MS23 was linear in the range from 50 to 10,000 ng/ml. The limit of quantification (LOQ) was 50 ng/ml. The results demonstrate that the method has linearity (R = 0.9989), specificity, and acceptable precision/accuracy. This method is simple, economic, and sufficient for in vivo pharmacokinetic studies on the compound.     

Linear electrical properties of isolated cardiac cells

Summary A frequency domain equivalent circuit analysis of isolated ventricular cells indicated the presence of an internal membrane structure which has a total capacitance four- to sixfold larger than the surface membrane. The internal membrane was mainly attributed to the sarcoplasmic reticulum since other morphological studies have shown that its area is many-fold larger than that of the surface membrane. Corresponding estimates from the transverse tubular system indicate an area less than that of the surface; thus this structure is not a likely candidate for the observed internal capacitance. Measurements in hypertonic solutions showed that the access resistance to the internal membrane reversibly increased as the tonicity was elevated. Freeze-fractured electron microscopic studies confirmed that hypertonic solutions increased the volume of transverse tubular system, which thus appears to have little relation to the access resistance. The most probable source of the access resistance is the diadic junction to the sarcoplasmic reticulum, which therefore would electrically couple it to the surface membrane.     

Proline metabolism in isolated rat liver cells

The metabolism of proline was studied in liver cells isolated from starved rats. The following observations were made. 1. Consumption of proline could be largely accounted for by production of glucose, urea, glutamate and glutamine. 2. At least 50% of the total consumption of oxygen was used for proline catabolism. 3. Ureogenesis and gluconeogenesis from proline could be stimulated by partial uncoupling of oxidative phosphorylation. 4. Addition of ethanol had little effect on either proline uptake or oxygen consumption, but strongly inhibited the production of both urea and glucose and caused further accumulation of glutamate and lactate. Accumulation of glutamine was not affected by ethanol. 5. The effects of ethanol could be overcome by partial uncoupling of oxidative phosphorylation. 6. The apparent K_m values of argininosuccinate synthetase (EC 6.3.4.5) for aspartate and citrulline in the intact hepatocyte are higher than those reported for the isolated enzyme. 7. 3-Mercaptopicolinate, an inhibitor of phosphoenolpyruvate carboxykinase (EC 4.1.1.32), greatly enhanced cytosolic aspartate accumulation during proline metabolism, but inhibited urea synthesis. 8. It is concluded that when proline is provided as a source of nitrogen to liver cells, production of ammonia by oxidative deamination of glutamate is inhibited by the highly reduced state of the nicotinamide nucleotides within the mitochondria. 9. Conversion of proline into glucose and urea is a net-energy-yielding process, and the high state of reduction of the nicotinamide nucleotides is presumably maintained by a high phosphorylation potential. Thus when proline is present as sole substrate, the further oxidation of glutamate by glutamate dehydrogenase (EC 1.4.1.3) is limited by the rate of energy expenditure of the cell.     

Heat acclimation: cardiac performance of isolated rat heart

Cardiac performance was studied in the isolated perfused hearts of rats heat acclimated at 34 degrees C (AC) and their age-matched controls (C). The pressure-volume curves during isovolumetric conditions showed a shift to the right in AC compared with C hearts. At similar left ventricular (LV) volumes end-diastolic and peak systolic pressures of AC hearts were lower, but no difference was observed in the maximal pressure developed at the highest LV volumes measured. In both C and AC hearts the developed force decreased as pacing rate increased. AC and C heart responses were the same up to 250 pulses/min. At higher frequencies the amplitude of the developed force of AC hearts was smaller than that of the controls. In accordance the tension produced by very early premature beat reduced in AC compared with C hearts. Since no hypertrophy was observed in AC hearts, it is concluded that heat acclimation results in a change in the intrinsic properties of the AC hearts exhibited by increased compliance, reduced chamber stiffness, and a decrease in the tension developed for each volume load. It is also suggested that at a high beating rate AC hearts fail to restitute its contractility as quickly as C hearts.     

5'-Nucleotidase activity of isolated mature rat cardiac myocytes

Specific location of 5'-nucleotidase in the heart has been uncertain, some authors citing evidence for an exclusively non-myocyte location, while other data point to the existence of cytoplasmic and membrane-bound fractions. Single myocytes isolated from mature rat heart, and free of endothelial or interstitial cells, have been used to establish that muscle cells of the myocardium are rich in 5'-nucleotidase, exhibiting activity sufficient to account for the total myocardial content of this enzyme. All 5'-nucleotidase is accessible to extracellular AMP. Inhibitors of 5'-nucleotidase and adenosine transport have been used to establish that only the adenosine component of adenine nucleotides is taken up by myocytes, but hydrolysis of AMP by 5'-nucleotidase does not commit the adenosine formed to transport across the sarcolemmal membrane. Myocytes also have ecto-phosphatases which hydrolyse ADP and ATP.     

Uniform sarcomere shortening behavior in isolated cardiac muscle cells

We have observed the dynamics of sarcomere shortening and the diffracting action of single, functionally intact, unattached cardiac muscle cells enzymatically isolated from the ventricular tissue of adult rats. Sarcomere length was measured either (a) continuously by a light diffraction method or (b) by direct inspection of the cell's striated image as recorded on videotape or by cinemicroscopy (120--400 frames/s). At physiological levels of added CaCl2 (0.5--2.0 mM), many cells were quiescent (i.e., they did not beat spontaneously) and contracted in response to electrical stimulation (less than or equal to 1.0-ms pulse width). Sarcomere length in the quiescent, unstimulated cells (1.93 +/- 0.10 [SD] micrometers), at peak shortening (1.57 +/- 0.13 micrometers, n = 49), and the maximum velocity of sarcomere shortening and relengthening were comparable to previous observations in intact heart muscle preparations. The dispersion of light diffracted by the cell remained narrow, and individual striations remained distinct and laterally well registered throughout the shortening- relengthening cycle. In contrast, appreciable nonuniformity and internal buckling were seen at sarcomere lengths < 1.8 micrometers when the resting cell, embedded in gelatin, was longitudinally compressed These results indicate (a) that shortening and relengthening is characterized by uniform activation between myofibrils within the cardiac cell and (b) that physiologically significant relengthening forces in living heart muscle originate at the level of the cell rather than in extracellular connections. First-order diffracted light intensity, extremely variable during sarcomere shortening, was always greatest during midrelaxation preceding the onset of a very slow and uniform phase of sarcomere relengthening.     

The lipoprotein lipase (clearing-factor lipase) activity of cells isolated from rat cardiac muscle.

The total lipoprotein lipase activity recovered in suspension of cells prepared from adult rat hearts was unaffected by the nutritional state of the animals used. The enzyme activity present in the cell suspensions was almost exclusively associated with the cardiac muscle cells present as the major cell type.     

Futile cycles in isolated perfused rat liver and in isolated rat liver parenchymal cells.

Isolated livers from fed rats were perfused with a medium containing glucose labeled uniformly with ¹⁴C and specifically with ³H. There was considerable formation of glucose from endogenous sources but simultaneously uptake of about half of the ¹⁴C in glucose. After 2 hours the ${}^3\text{H}{}^{14}\text{C}$ ratios in perfusate glucose decreased by 55–60% with (2-³H, U-¹⁴C), 40–50% with (5-³H, U-¹⁴C), 25–30% with (3-³H or 4-³H, U-¹⁴C) and by 10–15% with (6-³H, U-¹⁴C) glucose. Qualitatively comparable patterns were obtained with rat hepatocytes. These results demonstrate recycling of carbon between glucose and pyruvate. Superimposed upon this there is an extensive futile cycle between glucose and glucose 6-P. There is also futile cycling between fructose 6-P and fructose 1,6 P₂ and to a small extent between phosphoenol pyruvate and pyruvate.     

The fate of insulin in cardiac muscle. Studies on isolated muscle cells from adult rat heart.

Isolated muscle cells from adult rat heart were used to study myocardial degradation of insulin and the reactions after the initial binding event. After 60 min of association at 37 degrees C, 90% of specifically bound insulin could be dissociated from the cells; this fraction remained unaltered under steady-state conditions (up to 180 min). To assess the nature of cell-associated radioactivity, cardiocytes were solubilized and filtered on Sephadex G-50. After 5 min of association only intact insulin was observed, whereas under steady-state conditions 4% of 125I-labelled insulin bound to the cells was degraded to iodotyrosine-containing fragments. The Km for insulin degradation by isolated heart cells was estimated to be 1.75 x 10(-7)M. Receptor-mediated insulin degradation was studied by examination of the nature of radioactivity released by the cells after different times of association. After 5 min 83% of dissociating material consisted of intact insulin, whereas this fraction decreased to 50% under steady-state conditions. Treatment of cells with the lysosomotropic agent chloroquine (0.1 mM) significantly decreased the fraction that was eluted at the internal column volume. This study demonstrates that insulin degradation by the heart cell occurs by a receptor-independent and a receptor-dependent mechanism. The latter may involve internalization and a lysosomal pathway.     

L-Canavanine as a radiosensitization agent for human pancreatic cancer cells

This study evaluated the in vitro effect of L-canavanine on cell cycle progression in the two human pancreatic cancer cells lines PANC-1 and MIA PaCa-2. After 72 h of exposure to L-canavanine, the percentage of cells in the radiosensitive G₂/M phase of the cell cycle increased 6-fold in PANC-1 cells and 4-fold in MIA PaCa-2 cells, when compared to untreated cells. The capacity of L-canavanine to redistribute cells into the G₂/M phase of the cell cycle was both concentration- and time-dependent. Since many drugs that cause cells to accumulate in the G₂/M phase of the cell cycle are effective radiosensitization agents, the potential of L-canavanine to synergistically enhance the effects of ionizing radiation also was evaluated. The interaction between these treatment modalities was quantified using the median-effect equation and combination index analysis. L-Canavanine was found to be synergistic with radiation when either PANC-1 or MIA PaCa-2 cells were exposed to L-canavanine for 72 h prior to irradiation. These results suggest that L-canavanine in combination with radiation may have clinical potential in the treatment of pancreatic cancer.     

Electrophysiological properties of isolated rat liver cells

The electrophysiological properties of isolated rat liver cells were studied using the patch clamp method in whole-cell configuration. The membrane potential in isolated hepatocytes was -42 +/- 7 mV (n = 20). The input resistance (Rin) and the time constant (tau m) were 51 +/- 17 M (the range of 34 to 180 M omega) (n = 20) and 4.2 +/- 1.0 msec (the range of 3 to 16.5 ms) (n = 20). Assuming that the specific membrane capacitance is 1 microF/cm2, the membrane resistance and membrane capacitance were 42. +/- 9.0 K omega cm2 and 87 +/- 27 pF. These values indicate that isolated rat hepatocytes are not abnormally permeable or leaky. The current-voltage relationship was linear with no rectification. The depolarizing pulse from the resting potential did not induce fast or slow inward currents even when norepinephrine or high Ca2 (3.6 mM) were applied. This indicates that there is no voltage-sensitive Ca2+ channel in the isolated hepatocytes.