Effects of insulin upon ion transport

Effect of verapamil on insulin-stimulated choleresis

Insulin is one of several neurohumoral substances known to have a choleretic effect in vivo and in the isolated perfused rat liver. Infusion of insulin in the perfused rat liver preparation results in stimulation of bile acid-independent bile flow evidenced by increased bile flow, decreased bile acid concentration, and stable bile acid output. The mechanism of insulin-stimulated choleresis is unknown but may involve calcium as an intracellular second messenger. The present studies were performed to assess the role of membrane calcium channels in mediating choleresis and insulin-stimulated bile acid-independent bile flow in the in situ perfused rat liver. We have shown that verapamil, a specific calcium channel blocker, has no effect on bile flow, bile acid concentration, or bile acid output during bile acid-stimulated choleresis at a taurocholate infusion rate of 40 or 80 nmole/g liver/min. Insulin caused a significant increase in bile flow (18-30%) and a decrease in bile acid concentration (13-21%) without affecting bile acid output at a taurocholate infusion rate of 40 or 80 nmole/g liver/min. Verapamil failed to inhibit insulin-stimulated choleresis at a taurocholate infusion rate of 80 nmole/g liver/min. Although we observed an insulin-stimulated increase in bile flow and a decrease in bile acid concentration in the presence of verapamil at a taurocholate infusion rate of 40 nmole/g liver/min, these changes failed to reach statistical significance. We conclude that verapamil has no effect on choleresis or insulin-stimulated bile flow in the perfused rat liver and that the mechanism by which insulin promotes bile acid-independent bile flow is not mediated by verapamil-sensitive calcium channels.

Intracellular sodium content of a wall-less strain of Neurospora crassa and effects of insulin: a 23Na-NMR study

23Na-NMR has been used to investigate some factors influencing the sodium content of a wall-less strains of Neurospora crassa. The shift reagent Tm(DOTP)H2(NH4)3 proved useful for this purpose, while several other reagents, previously used by others, were found to be unsuitable for use with these cells. When the cells were grown, washed and resuspended in medium containing sodium (25.3 mM), the intracellular sodium concentration was calculated to be 11.9 +/- 1.4 mM. This value rose within two minutes of addition of glucose (100 mM), to greater than 14 mM. Preincubation of cells with insulin (100 nM) had a significant effect on the subsequent rate of sodium accumulation during the period 3-12 minutes following glucose addition. Insulin-treated cells showed a slow, continued accumulation of sodium during this period (+1.14 +/- 0.39%/min), while control cells lost sodium very slowly (-0.63 +/- 0.29%/min; P of difference = 0.005).

Effect of sepsis on amino acid transport system A and its response to insulin in incubated rat skeletal muscle

The effect of sepsis on neutral amino acid transport systems A, ASC, and L, was studied in incubated rat soleus (SOL) muscles. We also examined the effects of plasma from septic rats and of varying concentrations of insulin (10 to 10(5) microU/mL), added in vitro to incubated muscles, on system A amino acid transport. Sepsis was induced by cecal ligation and puncture (CLP) in rats weighing 40 to 60 g. Control rats were sham-operated. System A activity was assessed by determining uptake of 2-(methylamino)isobutyrate (MeAIB) 16 hours after CLP or sham-operation. System ASC was studied by measuring uptake of alpha-aminoisobutyric acid (AIB) in the presence of 25 mmol/L MeAIB and 25 mmol/L 2-amino-2-norbornane carboxylic acid (BCH) to inhibit uptake by systems A and L. System L activity was defined as sodium-independent uptake of cycloleucine. MeAIB uptake was reduced by 28% in muscles of septic rats, while amino acid transport by systems ASC and L was almost identical in muscles from control and septic rats. Addition of plasma from septic rats to incubated normal SOL muscles inhibited MeAIB uptake by 31%. Addition of insulin to the incubation medium resulted in increased uptake of MeAIB, both in nonseptic and septic muscle. The lowest hormone concentration tested that significantly enhanced MeAIB uptake in nonseptic muscle was 10(2) microU/mL and in septic muscle 10 microU/mL. The results suggest that sepsis in rats specifically inhibits amino acid transport system A and that reduced muscle amino acid uptake may be caused by a circulating factor in sepsis.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin resistance is a characteristic feature of primary hypertension independent of obesity

The relationship between abnormalities in carbohydrate metabolism and hypertension was studied in 143 newly detected hypertensive patients (59% obese) of both sexes (90 males, 53 females) and compared with 51 normotensive controls. Insulin-mediated glucose disposal assessed with the euglycemic insulin clamp technique was significantly decreased in both non-obese (7.2 +/- 2.1 mg/kg/min; P less than .05) and obese hypertensives (5.1 +/- 2.1 mg/kg/min; P less than .01) compared with the controls (8.4 +/- 1.8 mg/kg/min). The decrease in insulin sensitivity and increase in basal insulin as well as a decreased rate of glucose disposal after an intravenous glucose tolerance test (IVGTT) were verified also after statistical adjustment for sex, age, body mass index, and waist-hip ratio. The insulin index (ratio between peak and basal insulin) during IVGTT was significantly decreased in the hypertensive patients (P less than .001). After the statistical adjustment for the factors mentioned the following lipid abnormalities were still significant: total cholesterol (6.25 +/- 1.12 mmol/L non-obese; 6.06 +/- 1.20 mmol/L obese; 5.41 +/- 1.02 mmol/L controls), triglycerides (1.70 +/- 0.74 mmol/L nonobese; 2.26 +/- 1.13 mmol/L obese; 1.24 +/- 0.53 mmol/L controls) and free fatty acids (0.57 +/- 0.20 mmol/L nonobese; 0.59 +/- 0.20 mmol/L obese; 0.48 +/- 0.15 mmol/L controls). This study shows that after correction for a series of probable confounding variables, hypertension emerges as part of a syndrome characterized by major abnormalities of carbohydrate, insulin, and lipid metabolism, which independently or in concert may act as important risk factors for cardiovascular disease.

Molecular genetics of Na,K-ATPase

Researchers in the past few years have successfully used molecular-genetic approaches to determine the primary structures of several P-type ATPases. The amino-acid sequences of distinct members of this class of ion-transport ATPases (Na,K-, H,K-, and Ca-ATPases) have been deduced by cDNA cloning and sequencing. The Na,K-ATPase belongs to a multiple gene family, the principal diversity apparently resulting from distinct catalytic alpha isoforms. Computer analyses of the hydrophobicity and potential secondary structure of the alpha subunits and primary sequence comparisons with homologs from various species as well as other P-type ATPases have identified common structural features. This has provided the molecular foundation for the design of models and hypotheses aimed at understanding the relationship between structure and function. Development of a hypothetical transmembrane organization for the alpha subunit and application of site-specific mutagenesis techniques have allowed significant progress to be made toward identifying amino acids involved in cardiac glycoside resistance and possibly binding. However, the complex structural and functional features of this protein indicate that extensive research is necessary before a clear understanding of the molecular basis of active cation transport is achieved. This is complicated further by the paucity of information regarding the structural and functional contributions of the beta subunit. Until such information is obtained, the proposed model and functional hypotheses should be considered judiciously. Considerable progress also has been made in characterizing the regulatory complexity involved in expression of multiple alpha-isoform and beta-subunit genes in various tissues and cells during development and in response to hormones and cations. The regulatory mechanisms appear to function at several molecular levels, involving transcriptional, posttranscriptional, translational, and posttranslational processes in a tissue- or cell-specific manner. However, much research is needed to precisely define the contributions of each of these mechanisms. Recent isolation of the genes for these subunits provides the framework for future advances in this area. Continued application of biochemical, biophysical, and molecular genetic techniques is required to provide a detailed understanding of the mechanisms involved in cation transport of this biologically and pharmacologically important enzyme.

Recessive inheritance of a high number of sodium pump sites

The number of sodium pump sites on erythrocytes was measured on 1,847 individuals in 80 Utah kindreds ascertained through probands with cardiovascular disease. Likelihood analysis supported recessive inheritance of high pump number. The major locus explained 14.0% of the variance in pump number; polygenic inheritance explained another 63.4%. Homozygotes for the recessive allele occurred with a frequency of 1.74% and had a mean pump number estimated as 566.0 sites/red blood cell (RBC) versus a mean of 312.2 sites/RBC for the other genotypes. Young individuals with the high pump number genotype were leaner, and older adults with the high pump number genotype were heavier. Diabetes and early hypertension were more prevalent in women with the high pump number genotype. Although not significant, obesity in adults of both sexes and early coronary heart disease in men were more prevalent in individuals with the high pump number genotype.

In vitro Na+,K+-ATPase (EC 3.6.1.3)-dependent respiration and protein synthesis in skeletal muscle of pigs fed at three dietary protein levels

1. Eighteen pigs were offered diets containing 130, 170 or 210 g protein/kg with three barrows and three gifts per diet from 20 to 60 kg live weight. Oxygen consumption, Na+,K+-ATPase (EC 3.6.1.3)-dependent and -independent respiration and protein synthesis were measured in vitro in intercostal and sartorius muscle preparations from these pigs. 2. Increasing dietary protein concentration increased (P less than 0.01) daily gain and dissectible muscle in carcass. 3. O2 consumption and Na+,K+-ATPase-dependent respiration of the intercostal and sartorius muscles increased linearly (P less than 0.01) with increase in dietary protein concentration. The requirement for the support of the transport of Na+ and K+ across the cell membrane in these muscles, on average, accounted for 22-25% of the O2 consumption. 4. Synthesis rate (mg/g per d) of protein in the sartorius muscle increased (P less than 0.05) from 3.05 to 5.07 and increased (P less than 0.1) from 2.57 to 4.06 in the intercostal muscle as dietary protein increased from 130 to 210 g/kg diet. 5. Regression of Na+,K+-ATPase-dependent respiration against protein synthesis in each of intercostal and sartorius muscles showed a linear relation, an attestation of a close link between productive processes and auxiliary energy expenditure.

Apolipoprotein, low density lipoprotein subfraction, and insulin associations with familial combined hyperlipidemia. Study of Utah patients with familial dyslipidemic hypertension

Familial dyslipidemic hypertension (FDH) is a syndrome recently described from sibships selected for early familial hypertension and found to have one or more of three fasting lipid abnormalities [high triglycerides, low high density lipoprotein (HDL) cholesterol, high low density lipoprotein (LDL) cholesterol]. In further analyses of these same 131 hypertensive subjects, apolipoprotein A-I and B, fasting plasma insulin (adjusted for body mass index), and detailed anthropometrics were different in two subgroups of FDH. Of 63 FDH patients, 19 met the criteria for familial combined hyperlipidemia (FCHL); 44 did not, but still had high triglyceride and/or low HDL cholesterol levels. When compared to 20 normolipidemic hypertensive patients, the 19 hypertensive patients with FCHL had 196% higher very low density lipoprotein cholesterol (p = 0.0001), 33% higher apolipoprotein B (p = 0.0002), smaller LDL particles (p = 0.007), and 73% higher fasting insulin (p = 0.003), but no significant differences in body mass index or skinfold thicknesses. The other 44 FDH patients without FCHL had 33% lower HDL (p = 0.0001), with only 8% lower apolipoprotein A-I levels (p = 0.20); significantly higher subscapular skinfolds (p = 0.02), weights (p = 0.002), body mass index (p = 0.006), knee widths (p = 0.0007), and wrist circumferences (p = 0.0009); smaller, denser LDL subfractions (p = 0.001); and increased apolipoprotein B levels (p = 0.01) compared to the normolipidemic hypertensive group. Increased fasting insulin levels were similar to the normolipidemic group and significantly lower than the FCHL group after adjustment for body mass index, suggesting a relationship between obesity and fasting insulin levels only in the non-FCHL group. We conclude that FDH consists of at least two subgroups: 1) FCHL with high apolipoprotein B, small LDL particles, and increased fasting plasma insulin levels, and 2) a less well-defined residual having upper central obesity with low HDL cholesterol and high triglyceride levels. Elevated insulin levels found in both groups, but possibly originating through different physiological mechanisms, may provide the pathophysiological connections between dyslipidemia, obesity, and hypertension.

Modifications induced by diabetes on the physicochemical and functional properties of erythrocyte plasma membrane

Increasing evidence suggests that in experimental diabetes an impairment in Na+,K+-ATPase activity plays a central role in the pathophysiology of diabetic complications, while only a few data are available with regard to human subjects. We studied the erythrocyte membrane Na+,K+-ATPase activity and membrane fluidity in insulin-dependent and non-insulin-dependent diabetic subjects. A significant decrease in the enzyme activity and in fluorescence polarization values was found in both groups compared with normal subjects. Neither Na+,K+-ATPase activity nor membrane fluidity was found to be related to metabolic control, assessed by means of fasting blood glucose levels and HbA1c. On the contrary, a significant correlation was observed between Na+,K+-ATPase activity and membrane fluidity in both insulin-dependent and non-insulin-dependent diabetic subjects. The present work provides evidence that a reduction in the Na+,K+-ATPase activity is present in the plasma membranes of insulin-dependent and non-insulin-dependent diabetics. Furthermore, it suggests that the change in enzyme activity might be related to modifications in membrane fluidity.

Increased skeletal muscle Na/K-ratio in obese men, but not in women, with glucose intolerance

Obesity is associated with glucose intolerance. Glucose is to a large extent disposed in the skeletal muscle. Peripheral insulin resistance, as well as decreased enzymatic activity in the skeletal muscle, has been suggested in type II diabetes. Potassium is essential for such enzymatic reactions. In this study, obese men, but not women, with glucose intolerance tended to have a lower total body potassium per kg body weight, indicating a smaller muscle mass, than weight-matched normoglycaemic men. They also had a lower skeletal muscle potassium content per 100 g dry weight (P less than 0.05) and a higher muscle Na/K-ratio (P less than 0.05) compared with obese men with normal glucose tolerance. Muscle fat and muscle sodium content were higher in obese men than in women with the same body mass index (P less than 0.01). The muscle electrolyte changes can be explained by a decrease in the insulin mediated Na/k-pump activity across the cell membrane or a smaller number of insulin receptors on the skeletal muscle cell in patients with glucose intolerance.

Insulin regulation of glucose metabolism in HT29 colonic adenocarcinoma cells: activation of glycolysis without augmentation of glucose transport

The effects of insulin on glucose transport and metabolism were examined in cultured HT29 human colonic adenocarcinoma cells. The presence of glucose transporters was verified by D-glucose displaceable [3H]cytochalasin B binding. The Kd and Bmax values from cytochalasin B binding studies were 190 +/- 30 nM and 8.4 +/- 1.4 pmol/mg protein, respectively. Glucose transport determined with 3-O-methylglucose showed saturable kinetics with a Km of 5.8 +/- 0.4 mM and a Vmax of 0.047 +/- 0.003 mumol/mg protein per min at 25 degrees C. Moreover, in HT29 cells, two classes of insulin binding sites were detected in radioligand binding experiments. Although insulin failed to stimulate glucose transport, it was found to activate glycolysis in HT29 cells. Glucose consumption increased from 0.33 +/- 0.03 mumol/mg protein per h to 0.49 +/- 0.05 mumol/mg protein per h and lactate production was augmented from 0.67 +/- 0.04 mumol/mg protein per h to 0.87 +/- 0.06 mumol/mg protein per h in response to 10(-7) to 10(-5) M insulin. Insulin also enhanced mannose metabolism. Apart from these two hexoses, HT29 cells exhibited a surprisingly narrow substrate specificity. With the possible exception of glyceraldehyde, little lactate was produced from alternative substrates, including adenosine, inosine, ribose, deoxyribose, dihydroxyacetone, galactose and fructose either with or without insulin. Despite its limited utilization by the glycolytic pathway, adenosine was readily salvaged for de novo synthesis of adenine nucleotides. These findings suggest that insulin directly influences substrate utilization through the glycolytic pathway in HT29 cells without activating the glucose transport pathway.

Intracellular pH regulation in papillary muscle cells from streptozotocin diabetic rats: an ion-sensitive microelectrode study

Intracellular pH regulation was studied in papillary muscle from STZ-induced diabetic rat hearts. In control bicarbonate solution there was no difference between the steady-state pHi values recorded from diabetic or normal papillary muscle. The addition of insulin had no effect on the pHi of either group. The amplitude of NH4+-induced alkalinization and the time course of recovery from alkalinization were similar in both normal and diabetic muscles. In both preparations, the recovery from alkalinization was similarly delayed by the disulfonic stilbene DIDS. This suggests the participation of a Cl-/HCO3- exchange in the recovery from alkalosis in rat myocardial cells that is not changed by diabetes. On the other hand, the amplitude of the acidification induced by the withdrawal of NH4+ was markedly increased in diabetic papillary muscles as compared to normal muscles. Moreover, there was a marked slowing down of the recovery from acidosis in the diabetics. The amplitude of NH+4 withdrawal-induced acidification was increased equally by amiloride in both normal and diabetic muscles. These findings suggest that diabetes is associated with a change in the activity of the amiloride-sensitive Na+/H+ exchange.

Insulin and phorbol ester stimulate conductive Na+ transport through a common pathway

Insulin stimulates Na+ transport across frog skin, toad urinary bladder, and the distal renal nephron. This stimulation reflects an increase in apical membrane Na+ permeability and a stimulation of the basolateral membrane Na,K-exchange pump. Considerable indirect evidence has suggested that the apical natriferic effect of insulin is mediated by activation of protein kinase C. However, no direct information has been available documenting that insulin and protein kinase C indeed share a common pathway in stimulating Na+ transport across frog skin. In the present work, we have studied the interaction of insulin and phorbol 12-myristate 13-acetate (PMA), a documented activator of protein kinase C. Preincubation of skins with 1,2-dioctanoylglycerol, another activator of protein kinase C, increases baseline Na+ transport and reduces the subsequent natriferic response to PMA. Preincubation with PMA markedly reduces the subsequent natriferic action of insulin. This effect does not appear to primarily reflect PMA-induced internalization of insulin receptors. The insulin receptors are localized on the basolateral surface of frog skin, but the application of PMA to this surface is much less effective than mucosal treatment in reducing the response to insulin. Preincubation with D-sphingosine, an inhibitor of protein kinase C, also reduces the natriferic action of insulin. The current results provide documentation that insulin and protein kinase C share a common pathway in stimulating Na+ transport across frog skin. The data are consistent with the concept that the natriferic effect of insulin on frog skin is, at least in part, mediated by activation of protein kinase C.

Diuretics and hyperkalaemia in diabetic ketoacidosis

Diabetic ketoacidosis (DKA) often presents with hyperkalaemia. We investigated whether it was more likely in patients taking potassium-retaining diuretics. A retrospective survey of all patients (552 cases) presenting in DKA between 1974 and 1984 was undertaken. Initial biochemical data were compared for patients recorded as taking potassium-retaining diuretics (7 cases) at the time of presentation with those taking potassium-losing diuretics (13 cases), and age matched control groups were selected from those who presented in DKA but were not taking diuretics. There was no significant difference in initial serum potassium levels between the diuretic treated groups. The serum sodium was higher in the control group than in the potassium losing group (p = 0.045) and the serum urea significantly lower (p = 0.045). We conclude that potassium-retaining diuretics do not predispose to hyperkalaemia in diabetic ketoacidosis.

In vitro insulin action on erythrocyte glucose metabolism in normal and diabetic rats

Alloxan-induced diabetes in rats significantly impaired the capacity of the erythrocytes to metabolise glucose in vitro to either lactic acid or CO2. Both these metabolic activities were initially insensitive to insulin in normal as well as in diabetic animals; but became responsive when these cells were subjected to insulin and glucose 'starvation' for 1 h through incubation in their absence. This action of insulin in starved cells showed concentration dependence and required preincubation with the hormone prior to addition of glucose.

Inhibition of insulin receptors by vanadate and ouabain

Insulin binding studies were performed, using cells from 5 non-obese, non-diabetic subjects, on four separate days: 2 were paired control studies to demonstrate precision, and 2 other sets were binding studies in which one incubation solution was a control and the other contained either vanadate, (10(-4) M) or ouabain (10(-4) M). For both substances tracer binding of 125I insulin was reduced significantly, 27% by vanadate and 30% by ouabain. Furthermore, at all points on the binding curve these substances inhibited binding by 18-98%, in a pattern consistent with reduced receptor number. The concentrations of vanadate or ouabain which we used did not change cell volume or inhibit trypan blue dye exclusion, as an index of cell viability. Because vanadate and ouabain inhibit Na+K+ATPase and have largely dissimilar effects on a variety of cell systems, our observations may reflect specific involvement of Na+K+ATPase in binding or closely related processes.

Relationship between ionic surroundings and insulin actions on glucose transport and Na,K-pump in muscles

1. It is well known that insulin has various effects on glucose transport and the Na,K-pump in muscles. It is also known to have some effects on the membrane potential--in general, insulin induces a hyperpolarization of the membrane in muscles. Furthermore, it is suggested that the actions of insulin are modified by changes in ionic surroundings. 2. In this review article, the actions of ionic surroundings and insulin on glucose transport in muscles are discussed; in particular, the effects of changes in extracellular and/or intracellular concentrations of Na, K, Ca and H ions will be mentioned. 3. The actions of ionic surroundings and insulin on the Na,K-pump in muscles are discussed; in particular, the effects of changes in extracellular an/or intracellular concentrations of Na, K, Ca and H ions will be examined. 4. The relationship between the actions of ionic surroundings and insulin are discussed. 5. In particular, the effects of changes in ionic surroundings on the insulin-induced hyperpolarization of the membrane are discussed by relating it to the Na,K-pump function. The relationship between the insulin-induced change in membrane potential and glucose transport will be also mentioned.

Altered erythrocyte and plasma sodium and potassium in hypertension, a facet of hyperinsulinemia

Red blood cell sodium and potassium, plasma potassium, glucose and insulin responses to oral glucose load, serum urate, and plasma triglycerides were determined in a stratified subsample (n = 89) of a representative population sample (n = 1211), comprising 30 nonobese normotensive subjects with normal glucose tolerance (reference group) and 59 subjects representing each of the seven possible combinations of abnormal glucose tolerance, obesity, and hypertension. Rate of cation imbalance (red blood cell sodium greater than or equal to 7.0 mEq/L, potassium less than 92.5 mEq/L, or plasma potassium greater than or equal to 4.5 mEq/L) was 88.1% in subjects with abnormal tolerance, obesity, or hypertension, as compared with 40.0% in the reference group (p less than 0.001). These subjects were also characterized by significantly greater rates of insulin response: 60- and 120-minute postload levels of 100 mU/L or more (88.1 vs 46.7%), plasma triglycerides of 80 mg/dl or more (89.8 vs 53.3%) and serum uric acid of 5.5 mg/dl or more (61.0 vs 26.7%; p less than 0.001 for all). The rate of cation imbalance was significantly associated with each of these three biochemical correlates: insulin response (p less than 0.01), triglycerides (p less than 0.001), and urate (p less than 0.001). In the total population sample, the rate of untreated hypertension increased from 18% to 35% to 55.3% (p less than 0.001), with an increase in the number of biochemical correlates of cation imbalance in combination with glucose intolerance and obesity.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of detergents on Na+ + K+-dependent ATPase activity in plasma-membrane fractions prepared from frog muscles. Studies of insulin action on Na+ and K+ transport

The increase in Na+/K+ transport activity in skeletal muscles exposed to insulin was analysed. Plasma-membrane fractions were prepared from frog (Rana catesbeiana) skeletal muscles, and examination of the Na,K-ATPase (Na+ + K+-dependent ATPase) activity showed that it was insensitive to ouabain. In contrast, plasma-membrane fractions prepared from ouabain-pretreated muscles, by the same procedures, showed extremely low Na,K-ATPase activity. On adding saponin to the membrane suspension, the Na,K-ATPase activity increased, according to the detergent concentration. The maximum activity was about twice the control value, at 0.33 mg of saponin/mg of protein. Thus saponin makes vesicle membranes leaky, allowing ouabain in assay solutions to reach receptors on the inner surface of vesicles. Addition of insulin to saponin-treated membrane suspensions had no effect on the Na,K-ATPase activity, whereas the maximum activity of Na,K-ATPase in whole muscles was stimulated by exposure to insulin. The results show that the stimulation of Na+/K+ transport by insulin is not directly due to insulin binding to receptors on the cell surface, but rather support the view that the increase in the Na,K-ATPase induced by insulin requires an alteration of intracellular events.

Red cell membrane (Na+ +K+)-ATPase in diabetes mellitus

The red cell membrane (Na+ +K+)-ATPase activity is significantly elevated in diabetes mellitus. The osmotic fragility of diabetic red cells is also increased. In vivo insulin treatment restores the enzyme activity and the osmotic fragility to the normal level. In vitro insulin treatment of diabetic red cells was found to inhibit the further increase in its activity, but it failed to restore the activity to the normal level as in vivo. In diabetes increased Km of (Na+ +K+) ATPase for ATP was observed but Vmax remained the same. Arrhenius plot of this enzyme was also altered in diabetes.

Ionic dependence of amino-acid transport in the exocrine pancreatic epithelium: calcium dependence of insulin action

Rapid unidirectional transport (15 sec) of L-serine and 2-methylaminoisobutyric acid (MeAIB) was studied in the isolated perfused rat pancreas using a dual-tracer dilution technique. Time-course experiments in the presence of normal cation gradients revealed a time-dependent transstimulation of L-serine influx and transinhibition of MeAIB influx. Transport of the model nonmetabolized System A analog MeAIB was Na+ dependent and significantly inhibited during perfusion with 1 mM ouabain. Although transport of L-serine was largely Na+ independent, ouabain caused a time-dependent inhibition of transport. Influx of both amino acids appeared to be inhibited by the ionophore monensin but unaffected by a lowered extracellular potassium concentration. Removal of extracellular calcium had no effect on influx of the natural substrate L-serine, whereas stimulation of transport by exogenous insulin (100 microU/ml) was entirely dependent upon extracellular calcium and unaffected by ouabain. Paradoxically, exogenous insulin had no effect on the time-course of MeAIB influx. The characteristics of L-serine influx described in earlier studies together with our present findings suggest that insulin may modulate the activity of System asc in the exocrine pancreatic epithelium by a calcium-dependent mechanism.

Potassium homeostasis and hypokalemia

Potassium, largely an intracellular cation, contributes to the regulation of cellular volume, to tissue growth and metabolic synthesis of proteins and nucleic acids, and to the integrity of electrical properties of excitable tissues as well as nonexcitable, transporting epithelia. Potassium balance is closely regulated by a variety of nonrenal and renal mechanisms. When potassium losses are sufficient to induce hypokalemia, either through nonrenal or renal causes, profound adverse effects on neuromuscular, cardiac, vascular, and renal tissues may ensue. The diagnostic approach is straightforward, and therapy must be directed to replenish losses without inducing a rapid, excessive, and potentially fatal increase in the potassium concentration of the serum.

Insulin stimulates volume absorption in the rabbit proximal convoluted tubule

The present in vitro microperfusion study examined whether insulin affects volume absorption (Jv) in the proximal convoluted tubule (PCT). PCT were perfused with an ultrafiltrate-like solution and were bathed in a serum-like albumin solution. Addition of a physiologic concentration of 10(-10) M insulin to the bathing solution resulted in a stimulation of Jv and a more negative transepithelial potential difference (PD). There was a progressive stimulation of the lumen negative PD and Jv with higher insulin concentrations. Maximal stimulation occurred at 10(-8) M bath insulin. The insulin-induced stimulation of volume reabsorption was also observed when glucose and amino acids were removed from the luminal perfusate. Direct examination of the effect of insulin on glucose, chloride, and bicarbonate absorption demonstrated that the transport of all these solutes was stimulated by insulin. Addition of insulin to the luminal perfusate had no affect on Jv. These data show that insulin has a direct effect to stimulate Jv in the proximal tubule.

Activation of amino acid uptake at fertilization in the sea urchin egg. Requirement for proton compartmentalization during cytosolic alkalosis

The comparative importance of the release of intracellular ionic calcium, Na+/H+ exchange and cytosolic alkalosis as activator signals was studied on the development of amino acid uptake at fertilization in sea urchin eggs. We show that, once stimulated, the rate of valine uptake is greatly dependent upon intracellular pH. Suppression of the Na+/H+ exchange at the time of activation, by applying ionophore (A23187) in sodium-free artificial sea water (ONaASW), inhibits the development of valine influx. This cannot be restored by a further (30 min later) alkalosis by transferring eggs into sea water. Suppressing the alkalosis in the presence of Na+/H+ exchange at fertilization by simultaneous addition of acid into sea water results in activation of the amino acid carrier which exhibits an increased rate of transport as soon as the eggs are replaced in sea water at pH 8.0. The absence of alkalosis in eggs activated in ONaASW can be counterbalanced either by adding NH4Cl 10 mM or by transfer into ASW at pH 9.0 at activation. Ammonia-treated eggs absorbed amino acid as controls, whereas eggs in sea water at pH 9.0 failed to develop a valine uptake system, suggesting that ammonia can completely replace the effect of Na+/H+ exchange. Furthermore, addition of NH4Cl immediately before fertilization conceals the Na+/H+ exchange but stimulates valine uptake as in controls. These data suggest that: the occurrence of the intracellular calcium increase alone is not sufficient for the develpment of the amino acid transport system; cell alkalinization at fertilization derives from the cytoplasmic membrane-located Na+/H+ exchange and an inward movement of protons into a cortical acidic compartment, which is discussed.

Protein kinase C is not required for insulin stimulation of hexose uptake in muscle cells in culture

The L6 skeletal muscle cell line has been identified as a suitable model to study the action of insulin on glucose uptake in muscle [Klip, Li & Logan (1984) Am. J. Physiol. 247, E291-E296]. The signals that transfer information from occupied insulin receptors to glucose transporters remain unknown. Here we report that activation of protein kinase C by exogenous phorbol esters results in stimulation of glucose uptake. Protein C kinase activity was induced to migrate from the cytosolic fraction to the microsomal fraction after 40 min of exposure of intact cells to 4 beta-phorbol 12,13-dibutyrate. In contrast, incubation with insulin did not alter the subcellular distribution of the kinase. Prolonged preincubation of L6 cells with phorbol esters resulted in depletion of kinase C activity, whereas neither the basal rate of glucose uptake nor its stimulation by insulin were affected. This suggests that protein kinase C is expressed in L6 cells, and that insulin stimulation of hexose transport does not involve protein kinase C.

Calcium, cyclic AMP and protein kinase C--partners in mitogenesis

Evidence is steadily mounting that the proto-oncogenes, whose products organize and start the programs that drive normal eukaryotic cells through their chromosome replication/mitosis cycles, are transiently stimulated by sequential signals from a multi-purpose, receptor-operated mechanism (consisting of internal surges of Ca2+ and bursts of protein kinase C activity resulting from phosphatidylinositol 4,5-bisphosphate breakdown and the opening of membrane Ca2+ channels induced by receptor-associated tyrosine-protein kinase activity) and bursts of cyclic AMP-dependent kinase activity. The bypassing or subversion of the receptor-operated Ca2+/phospholipid breakdown/protein kinase C signalling mechanism is probably the basis of the freeing of cell proliferation from external controls that characterizes all neoplastic transformations.

Regulatory and energetic role of Na+ in amino acid uptake by fertilized sea urchin eggs

Relationships between the Na+ dependent amino acid uptake displayed by fertilized sea urchin eggs and the electrochemical gradient of Na+ was investigated. The time course of Na+ content and valine or alanine uptake was simultaneously monitored in Na+ loaded eggs [by fertilization in K+-free artificial sea water (OK-ASW), or by using monensin, antimycin, cyanide, or ciguatoxin]. Our results demonstrate that the uphill amino acid uptake follows the "Na+ gradient hypothesis." Subsequent fertilization of eggs Na+ depleted by ammonia for 40 min stimulates to a great extent the development of amino acid uptake as compared with controls eggs. By using simultaneous change of external and intracellular Na+ concentration, we studied the specific role of this ion. An increase in internal Na+ inhibits the uptake through trans inhibitory action while an increase in external Na+ stimulates the efficiency of the uptake system. In eggs fertilized since 30 min, hyperpolarization obtained in K+-free ASW stimulates amino acid uptake while depolarization (transfer from K+ free ASW to ASW) inhibits it. This potential-dependent effect developed after fertilization with a time course similar to that the establishment of K+ conductance described by R. A. Steinhardt, L. Lundin, and D. Mazia (1971, Proc. Natl. Acad. Sci. USA 68, 2426-2430). In conclusion, our results point out that slight modulations in the activity of the Na+ pump can widely affect the amino acid uptake, suggesting that activation of Na+/K+ ATPase has a key role in the stimulation of amino acid transport.

Effects of insulin on the total number of [3H]ouabain binding sites in normal human lymphocytes and after stimulation in vitro with concanavalin A

The effects of insulin in vitro on the total number of [3H]ouabain binding sites were studied in normal and concanavalin A-stimulated human T-lymphocytes after incubation for 18 and 42 h with 0, 10(2) and 10(5) mIU/l of insulin. While no effect on [3H]ouabain binding could be demonstrated in non-stimulated cells, a significant decrease could be produced in concanavalin A-stimulated cells. Mean +/- SD for the total number of [3H]ouabain binding sites per cell after 18 h incubation time with concanavalin A in the absence of insulin was 46,099 +/- 6,620 as compared with 44,783 +/- 8,347 in the presence of 10(2) mIU/l of insulin (non-significant) and 42,406 +/- 7,066 in the presence of 10(5) mIU/l (p = 0.031). The corresponding 42-h values were 47,075 +/- 9,412, 43,761 +/- 9,273 (p = 0.033) and 43,824 +/- 9,312 (p = 0.005).

(Na+ +K+)-ATPase activity in kidney basolateral membranes of non insulin dependent diabetic rats

Insulin resistant, Type II diabetes mellitus (NIDD) in a rat animal model results in profound changes in basal and insulin-stimulated membrane (Ca2+ +Mg2+)-ATPase activity in kidney basolateral membrane (BLM) preparations. We find that NIDD in these animals does not result in similar changes in membrane (Na+ +K+)-ATPase activity. Basal enzyme activity was the same in diabetic and control animals. Insulin treatment of diabetic animals in vivo resulted in hyperinsulinemia and increased BLM (Na+ +K+)-ATPase, while food restriction for 18 hr resulted in lowered enzyme activity. There was no direct effect of insulin on (Na+ +K+)-ATPase activity in isolated membranes from any of the animal groups. Thus, physiologic perturbations which alter insulin sensitivity and glucose homeostasis are accompanied by altered levels of (Na+ +K+)-ATPase activity. Lower levels of this membrane enzyme activity appear to be associated with optimal insulin action.

Insulin effects on human red blood cells

The effect of insulin on human red blood cells was investigated, both on intact cells and on isolated plasma membranes, testing the responsiveness of membrane-bound enzymes--such as (Na+-K+)-ATPase and 5'-nucleotidase--as well as the ouabain binding and ionic fluxes. It appears that insulin stimulates Na-pumping mechanisms increasing (Na+-K+)-ATPase activity through an enhanced availability of pumping sites, as can be inferred from the increased ouabain binding. The apparent unresponsiveness of fluorescence polarization parameters, following insulin treatment of isolated plasma membranes and intact cells, rules out--at present--an involvement of membrane lipid fluidity in the mechanism of action of insulin on human erythrocytes.

Induction of distinct types of spontaneous electrical activities in mammary epithelial cells by epidermal growth factor and insulin

Electrophysiological measurements of the membrane potentials of mouse mammary epithelial cells in primary culture revealed the presence of spontaneous-oscillating-hyperpolarizing potentials in cells incubated with epidermal growth factor. The hyperpolarizing potentials were 5-20 mV in amplitude and about 10 sec in duration. The peak height of the response was reduced by hyperpolarization, and the input membrane resistance decreased during the response. The response was probably due to activation of K+ channels. The latency period for the epidermal growth factor induction of the hyperpolarizing potential was approximately 3 hr. In contrast, insulin induced spontaneous-depolarizing potentials that were about 5 mV in amplitude and 1 sec in duration. The depolarizing potentials were attributed to activity of ion channels, since the peak height was dependent on the membrane potential and the depolarizing potential was accompanied by a decrease of input membrane resistance. The time lag for the induction of the depolarizing potential was 6-12 hr. Other hormones involved in mammary cell differentiation, such as cortisol and prolactin, neither induced the depolarizing potentials nor changed the induction of depolarizing potential by insulin. In addition, other growth factors, such as nerve growth factor and fibroblast growth factor, elicited no electrical activity.

Glucose transport and metabolism in rat renal proximal tubules: multicomponent effects of insulin

Glucose transport and metabolism, and the effect of insulin thereon, was studied using suspensions of rat renal tubules enriched in the proximal component. [U-14C]Glucose oxidation is a saturable process (Km 3.1 +/- 0.2 mM; Vmax 14 +/- 0.2 mumole 14CO2 formed/g tissue protein per h). Glucose oxidation and [14C]lactate formation from glucose are inhibited in part by phlorizin and phloretin: the data suggest that the rate-limiting entry of glucose into the cell metabolic pool occurs by both the Na-glucose cotransport system (at the brush border) and the equilibrating, phloretin-sensitive system (at the basal-lateral membrane). Raising external glucose from 5 to 30 mM markedly increases aerobic and anaerobic lactate formation. Gluconeogenesis from lactate is not affected by variations of glucose concentrations. 24 h after streptozotocin administration, aerobic lactate formation is enhanced, as is the uptake of methyl alpha-D-glucoside by the tubules, while anaerobic glycolysis is depressed. Streptozotocin treatment (ST) increases both the Km and Vmax of glucose oxidation; gluconeogenesis and lactate oxidation are not affected. The effect of streptozotocin treatment on lactate formation are abolished by 1 mU/ml insulin. Streptozotocin treatment increases tissue hexokinase activity, decreases glucose-6-phosphatase, but has no significant effect on fructose-1,6-diphosphatase; phosphoenolpyruvate carboxykinase and pyruvate dehydrogenase. The data demonstrate fast streptozotocin-induced changes in cellular enzymes of carbohydrate metabolism. The enhancing effect of streptozotocin on methyl alpha-glucoside uptake is transient: 8 days after administration of the agent, no significant difference from controls is found. It is concluded that under the given experimental conditions insulin enhances the equilibrating glucose entry by the phloretin-sensitive pathway at the basal-lateral membrane, and transiently inhibits the Na-glucose cotransport system.

Aldose reductase, glomerular metabolism, and diabetic nephropathy

To explore a possible link between diabetic nephropathy and the enhanced activity of the polyol pathway known to occur in diabetes, we examined several pertinent metabolic parameters in glomeruli isolated from control and streptozotocin-diabetic rats and assessed whether changes observed in diabetic glomeruli could be prevented by the oral administration of the aldose reductase inhibitor sorbinil. We found that glomerular polyol content is significantly increased in diabetes, whereas glomerular myo-inositol content is significantly reduced. The sorbitol accumulation and myo-inositol depletion were both completely prevented by sorbinil, which was given throughout the duration of diabetes. Activity of the membrane-bound sodium/potassium adenosine triphosphatase (Na-K-ATPase) was decreased in diabetic samples; this change was also completely prevented by sorbinil. Erythrocyte deformability is another factor that has been implicated in the pathogenesis of microangiopathic complications. The ability of red blood cells to undergo an adaptation in shape that permits passage through the smallest vessels is impaired in diabetes. Using blood from control, diabetic, and sorbinil-treated diabetic rats, we found that erythrocyte deformability was decreased in diabetic samples and that sorbinil treatment significantly improved this parameter. Thus, if the glomerular consequences of sorbitol accumulation, myo-inositol depletion, reduced Na-K-ATPase activity, and decreased erythrocyte deformability are pathogenetically implicated in diabetic nephropathy, the ability of sorbinil to impact on these changes suggests that it could favorably impact on the nephropathic process.

Insulin stimulates renal glomerular sodium-potassium adenosine triphosphatase activity

The effect of insulin on total and ouabain-inhibited membrane-bound adenosine triphosphatase (ATPase) activity in renal glomeruli isolated from adult white rats was examined. In concentrations of 1-10 micrograms/ml, insulin significantly stimulated the ouabain-inhibited (Na+ + K+)-ATPase activity, without affecting total (composite) ATPase activity. These results, coupled with previous findings demonstrating that glomerular (Na+ + K+)-ATPase activity is reduced in acute streptozotocin diabetes, suggest that the renal glomerulus is a target tissue with respect to this biologic effect of insulin.

Sodium-hydrogen exchange and glucose transport in renal microvillus membrane vesicles from rats with diabetes mellitus

Diabetes mellitus is associated with important changes in renal hemodynamics and transport function. Disturbances in solute transport have also been characterized in nonrenal tissues during hyperglycemia and insulinopenia. The purpose of this study was to determine if diabetes is associated with adaptive changes in function of the brush-border membrane of the proximal tubule. We studied Na+ and glucose transport in rat microvillus membrane vesicles isolated from the renal cortex of streptozotocin-induced and BB/W autoimmune diabetic rats. Untreated diabetes was associated with an increase in pH-stimulated total and amiloride-sensitive 22Na+ uptake into vesicles. Insulin treatment returned vesicle 22Na+ uptake to control levels. The increased Na+/H+ exchange was shown to be a result of increased net renal acid production rather than a specific response to insulinopenia because treatment with NaHCO3 also returned 22Na+ uptake to control levels. On the other hand, Na+-glucose cotransport, which was depressed in vesicles from untreated diabetics, returned to control levels with insulin but not NaHCO3 administration. This decreased Na+-glucose cotransport was not secondary to reduction in transport sites in untreated diabetics. These results show that in diabetes mellitus, increased Na+/H+ exchange activity is not the direct result of insulinopenia. However, the diabetic state appears to alter the functioning of the luminal Na+-glucose cotransporter.

Studies on the specific activity of [gamma-32P]ATP in adipose and other tissue preparations incubated with medium containing [32P]phosphate

The specific activity of the gamma-32P position of ATP was measured in various tissue preparations by two methods. One employed HPLC and the enzymatic conversion of ATP to glucose 6-phosphate and ADP. The other was based on the phosphorylation of histone by catalytic subunit of cAMP-dependent protein kinase (Hawkins, P.T., Michell, R.H. and Kirk, C.J. (1983) Biochem. J. 210, 717-720). The HPLC method also allowed the incorporation of 32P into the (alpha + beta)-positions of ATP to be determined. In rat epididymal fat-pad pieces and fat-cell preparations the specific activity of [gamma-32P]ATP attained a steady-state value after 1-2 h incubation in medium containing 0.2 mM [32P]phosphate. Addition of insulin or the beta-agonist isoprenaline increased this value by 5-10% within 15 min. Under these conditions the steady-state specific activity of [gamma-32P]ATP was 30-40% of the initial specific activity of the medium [32P]phosphate. However, if allowance was made for the change in medium phosphate specific activity during incubations the equilibration of the gamma-phosphate position of ATP with medium phosphate was greater than 80% in both preparations. The change in medium phosphate specific activity was a combination of the expected equilibration of [32P]phosphate with exchangeable intracellular phosphate pools plus the net release of substantial amounts of tissue phosphate. At external phosphate concentrations of less than 0.6 mM the loss of tissue phosphate to the medium was the major factor in the change in medium phosphate specific activity. It is concluded that little advantage is gained in employing external phosphate concentrations of less than 0.6 mM in experiments concerned with the incorporation of phosphate into proteins and other intracellular constituents. Indeed, a low external phosphate concentration may cause depletion of important intracellular phosphorus-containing components.