Ouabain-resistant hyperpolarization induced by insulin in aggregates of embryonic heart cells

Insulin increases the turnover rate of Na+-K+-ATPase in human fibroblasts

Insulin stimulates K+ transport by the Na+-K+-ATPase in human fibroblasts. In other cell systems, this action represents an automatic response to increased intracellular [Na+] or results from translocation of transporters from an intracellular site to the plasma membrane. Here we evaluate whether these mechanisms are operative in human fibroblasts. Human fibroblasts expressed the alpha(1) but not the alpha(2) and alpha(3) isoforms of Na+-K+-ATPase . Insulin increased the influx of Rb+, used to trace K+ entry, but did not modify the total intracellular content of K+, Rb+, and Na+ over a 3-h incubation period. Ouabain increased intracellular Na+ more rapidly in cells incubated with insulin, but this increase followed insulin stimulation of Rb+ transport. Bumetanide did not prevent the increased Na+ influx or stimulation of Na+-K+-ATPase. Stimulation of the Na+-K+-ATPase by insulin did not produce any measurable change in membrane potential. Insulin did not affect the affinity of the pump toward internal Na+ or the number of membrane-bound Na+-K+-ATPases, as assessed by ouabain binding. By contrast, insulin slightly increased the affinity of Na+-K+-ATPase toward ouabain. Phorbol esters did not mimic insulin action on Na+-K+-ATPase and inhibited, rather than stimulated, Rb+ transport. These results indicate that insulin increases the turnover rate of Na+-K+-ATPases of human fibroblasts without affecting their number on the plasma membrane or modifying their dependence on intracellular [Na+].

Type I and II models of diabetes produce different modifications of K+ currents in rat heart: role of insulin

1. Several K+ currents were measured and compared in enzymatically dispersed ventricular myocytes from control and diabetic rats. 2. Diabetic conditions were established either with a single intravenous injection of streptozotocin (STZ, 100 mg kg-1; 6-14 days duration) or by feeding with a fructose-enriched diet for 4-10 weeks. Both groups became hyperglycaemic, with the former having decreased and the latter having elevated levels of plasma insulin. These conditions therefore mimic type I (insulin-dependent) and type II (non-insulin-dependent) diabetes mellitus, respectively. 3. As reported previously, a Ca(2+)-independent transient outward K+ current, I(t), was attenuated in the type I model. This was not observed in the type II model. The two models differed greatly in the changes observed in a quasi-steady-state K+ current denoted Iss. In the STZ model Iss was substantially attenuated, whereas in the fructose-fed model it was augmented. In both models, the background inwardly rectifying current, IK1, was unchanged. Concomitantly, there was a substantial prolongation of the action potential in the STZ model but not in the fructose-fed model. 4. Incubation of control myocytes with insulin (100 nM) for 5-9 h caused a significant augmentation of Iss, with no effect on I(t) or on IK1. Incubation of myocytes from STZ-diabetic rats with insulin reversed the attenuation of I(t), but not of Iss. 5. The effect of insulin was not blocked by wortmannin, an inhibitor of phosphatidylinositol 3-kinase. However, inhibition of the mitogen-activated protein kinase pathway with PD98059 prevented restoration of I(t). Insulin action on I(t) may therefore involve changes in transcription or expression of channel proteins, rather than changes in cellular metabolism.

Short-term action of insulin on Aplysia neurons: generation of a possible novel modulator of ion channels

In mollusks as in other animals, peptides can act as hormones, growth factors, and neurotransmitters. The presence of insulin in vertebrate brain as well as its actions on nerve cells led us to examine the electrophysiological effects of the mammalian hormone on Aplysia neurons. Application of insulin extracellularly causes hyperpolarization of L14 and L10, identified neurons of the abdominal ganglion. This hyperpolarization is associated with a decreased membrane conductance that reverses at -35 mV. We also injected inositol phosphate glycan (IPG) into the identified neurons. This complex sugar, which was purified from rat liver and which is a putative second messenger for insulin in nonneural vertebrate cells (Saltiel and Cuatrecasas, 1986; Saltiel, Osterman, and Darnell, 1988), causes hyperpolarization with decreased membrane conductance in L14 and L10 similar to the effects of insulin. Furthermore, exposure of isolated ganglia to insulin results in the generation of IPG with a compensating decrease in its glycosyl-phosphatidylinositol precursor. We suggest that, in addition to its other roles, insulin may function as a neuropeptide transmitter using IPG as a second messenger.

Molecular genetics of severe insulin resistance

Leprechaunism and type A diabetes represent inborn errors of insulin resistance whose phenotypes suggested causation by mutations in the insulin receptor gene. Cells cultured from patients with leprechaunism specifically lacked high-affinity insulin binding. Partial but different degrees of impairment were observed in cells cultured from first-degree relatives. Different mutations in the insulin receptor's alpha subunit were proposed in different families (Ark-1, Atl, Minn, Mount Sinai) based on phenotype, cellular insulin binding, and insulin receptor structure. Molecular cloning and sequencing of mutant insulin receptor cDNA from family Ark-1 confirmed that the proband inherited a maternal missense and a paternal nonsense mutation in the alpha subunit and was a compound heterozygote. The insulin receptor was immunologically present on the plasma membrane of fibroblasts cultured from patients Ark-1 and Atl but was markedly reduced in cells from patients Minn and Mount Sinai. In cells from patient Minn, but not from patient Mount Sinai, the decreased number of insulin receptors was associated with reduced insulin receptor mRNA. In two families with the less severe form of insulin resistance, type A diabetes, mutations altered post-translational processing of the insulin receptor molecule. At a cellular level, these mutations of the alpha subunit of the insulin receptor shared defective binding and impaired stimulation of sugar transport by insulin. In family Atl, however, glucose uptake was constitutively increased. Thus, genetic variation in the insulin receptor gene causes a spectrum of inherited insulin-resistant syndromes and altered cellular signaling.

Effects of insulin and epinephrine on Na+-K+ and glucose transport in soleus muscle

To identify possible cause-effect relationships between changes in active Na+-K+ transport, resting membrane potential, and glucose transport, the effects of insulin and epinephrine were compared in rat soleus muscle. Epinephrine, which produced twice as large a hyperpolarization as insulin, induced only a modest increase in sugar transport. Ouabain, at a concentration (10(-3) M) sufficient to block active Na+-K+ transport and the hyperpolarization induced by the two hormones, did not interfere with sugar transport stimulation. After Na+ loading in K+-free buffer, the return to K+-containing standard buffer caused marked stimulation of active Na+-K+ transport, twice the hyperpolarization produced by insulin but no change in sugar transport. The insulin-induced activation of the Na+-K+ pump leads to decreased intracellular Na+ concentration and hyperpolarization, but none of these events can account for the concomitant activation of the glucose transport system. The stimulating effect of insulin on active Na+-K+ transport was not suppressed by amiloride, indicating that in intact skeletal muscle it is not elicited by a primary increase in Na+ influx via the Na+/H+-exchange system.

Insulin stimulation of an electrogenic pump at high extracellular potassium concentration

There is no agreement about the immediate mechanism by which insulin hyperpolarizes skeletal muscle, adipocytes, and myocardium. Of three candidates, one has been eliminated; the hyperpolarization is not secondary to an increase in intracellular [K]. There are reports that insulin hyperpolarizes by increasing relative permeability to K compared with that to Na ions, and other reports that insulin stimulates an ouabain-sensitive electrogenic Na-K exchange pump. Our evidence has been interpreted to support the former and deny the latter, when rat skeletal muscle is bathed at normal [K]. Crucial evidence for the latter has not been reported: insulin hyperpolarizes to a potential more negative than the K equilibrium potential. We now report that when rat caudofemoralis muscle is incubated with insulin at normal extracellular [K], then depolarized by increasing extracellular [K] to 38.4 mM, by equimolar substitution of KCl for NaCl, there is hyperpolarization compared with potentials of muscles treated similarly with respect to [K] but without insulin. Under these circumstances, the membrane potential in the presence of insulin is more negative than the new K equilibrium potential, and, in contrast to our previous experience with muscles bathed only in normal [K], the hyperpolarization in high [K] is reduced or eliminated by ouabain.

Insulin-specific receptor-mediated slowing of beat rate in embryonic heart cells

Spheroidal aggregates of embryonic heart cells showed their spontaneous beat rate when exposed to insulin. The concentration that produced a half-maximal response (1.7 nM) corresponded to the dissociation constant of binding to a specific high-affinity insulin receptor. The pace-maker phase of action potentials recorded during insulin perfusion was preceded by a prolonged or flattened after hyperpolarization, and its slope was less steep than controls. The action potential duration was also prolonged. These results indicate that physiological concentrations of insulin can regulate the embryonic heart rate.

Concanavalin A increases spontaneous beat rate of embryonic chick heart cell aggregates

The plant lectin concanavalin A (Con A), at concentrations of 5-200 micrograms/ml, induced a twofold to fivefold increase in spontaneous beat rate of cultured aggregates of ventricular cells from seven-day chick embryos. This response was time, dose, and temperature dependent and was accompanied by a decrease in transmembrane potential. It could be blocked or reversed by alpha-methyl-D-mannoside but was not reversed by dilution alone. Binding of the lectin occurred in the cold, but a temperature-dependent process was also necessary to produce the response. Divalent (succinyl) Con A did not cause a beat rate increase. Whole heart aggregates responded similarly but less intensely than ventricular aggregates. Atrial aggregates, and whole heart aggregates treated with 5 microgram/ml of Con A, produced a biphasic chronotropic response, first decreasing then increasing their beat rates. These results suggest that saccharide-bearing macromolecules on the heart cell surface play a role in regulating spontaneous beat rate.

Mechanism by which metabolic inhibitors depolarize cultured cardiac cells

To elucidate the means by which metabolic inhibition depolarizes cardiac cells, spontaneously beating chicken embryonic myocardial cell aggregates were voltage clamped during superfusion with 2,4-dinitrophenol and iodoacetic acid. In aggregates continuously clamped in the pacemaker potential range, abrupt exposure to these metabolic inhibitors produced a slow transient inward current. This inward current was not due to an alteration of the pacemaker current, IK2, because it could still be elicited after IK2 was abolished by Cs+ ions. The inward current was increased by hyperpolarization and decreased by depolarization. It became larger and more sustained if intermittent action potentials were allowed during exposure or if the aggregates were pretreated with either 10 mM Ca2+ or 2.7 microM acetylstrophanthidin. The inward current was suppressed by removal of extracellular Na+ or Ca2+. These observations suggest that early depolarization of cultured cardiac cells by metabolic inhibitors involves some of the same mechanisms as the transient inward current of digitalis toxicity--specifically, an effect of intracellular Ca2+ ions on membrane permeability. Similar phenomena could occur during other forms of metabolic inhibition such as myocardial ischemia.

Intracellular Na+ and K+ activities during insulin stimulation of rat soleus muscle

The action of insulin on the resting membrane potential (Em) and intracellular sodium and potassium activities (aNa, aK) of rat soleus muscle fibers was determined by direct intracellular measurements of aNa, aK, and Em using Na-selective, K-selective, and conventional microelectrodes. The use of these microelectrodes allowed us to continuously monitor these parameters in the same fiber. Although we were able to accurately measure aNa and aK and continuously monitor their levels throughout periods of insulin stimulation of up to 20 min duration, we were unable to detect any significant change in Em, aNa, or aK. Varying the concentration of insulin or extracellular glucose failed to alter our observations. These results indicate that the action of insulin on the sarcolemma and subsequent increase in glucose transport must result from some mechanism independent of a change in membrane potential or intracellular sodium or potassium activity.

Insulin does not hyperpolarize rat muscle by means of a ouabain-inhibitable process

Experiments were designed to test the hypothesis that insulin-induced hyperpolarization of rat skeletal muscle is mediated by stimulation of a ouabain-inhibitable electrogenic pump. Parallel experiments were carried out on rat caudofemoralis with isoproterenol, known to hyperpolarize rat skeletal muscle by stimulation of such a pump. Ouabain (10(-5) M) completely inhibited isoproterenol-induced hyperpolarization within 15 min but had no effect on half-maximal insulin-induced hyperpolarization. Ouabain (10(-6) M) inhibited isoproterenol effect by 60% during a period of 5-15 min. Ouabain (10(-4) M) had no effect on insulin-induced hyperpolarization within 10 min but depolarized during the next 10 min. In a separate series of studies in rat extensor digitorum longus muscle, 10(-5) M ouabain increased intracellular Na+ within 14 min. It is concluded that in rat caudofemoralis muscle, insulin-induced hyperpolarization is not mediated by a ouabain-inhibitable electrogenic pump.