Ability of insulin to increase calcium binding by adipocyte plasma membranes.

Abstract

Calcium specifically binds to adipocyte plasma membranes, demonstrating two classes of binding sites having affinity constants of 4.5 X 104 M-and 2.0 X 1O0 M-'. Insulin (100 microunits/ml) added directly to the isolated plasma membranes caused no alteration in calcium binding, whereas incubation of the adipocytes with 100 microunits/m of insulin resulted in a 25.0 k 1.6% increase in calcium binding to the subsequently isolated plasma membranes. The increase in calcium binding produced by insulin resulted from an increase in the maximum binding capacities of both classes of binding sites without alteration in their affinity constants. Additionally, a second pool of calcium in adipocyte plasma membranes has been identified by atomic absorption analysis; it was more than two times larger than the maximum binding capacity of the calcium binding system. This pool of calcium was stable, did not participate in the 45Ca2+ exchange, and was unaltered by insulin treatment. A similar stable pool of magnesium exists in plasma membranes and was also unaffected by insulin treatment. The increased capacity of the isolated plasma membranes to bind calcium after insulin treatment of the cells may represent an important bioregulating mechanism and supports the concept that calcium may play an important role in the effector system for insulin.The metabolic effects of insulin are thought to be produced by the interaction of insulin with its receptors on the plasma membranes (1, 2). This concept necessitates the involvement of a second messenger or an effector system to explain the mechanism of the complex cellular pleiotypic response to insulin. Considerable evidence suggests that adenosine 3':5'-cyclic monophosphate (cyclic AMP) does not fulfill the criteria of a second messenger for insulin in adipocytes (3, 4) and other cells (5, 6). Alternatively, the divalent cations, calcium and magnesium, have been proposed as possible second messengers for insulin (7-10), with a large body of indirect evidence to support this theory. Krahl has reported that insulin caused an increased intracellular magnesium concentration in rat hemiuteri (11) and adipose tissue (12) and that calcium and magnesium were necessary for maximum insulin stimulation of protein synthesis (12). Agents which promote an increased concentration of intracellular calcium, such as ouabain, procaine, lanthanum, and calcium ionophores, have been reported to mimic the antilipolytic action of insulin (9, 13, 14). Insulin was found to alter efflux of 45Ca2+ from preloaded fat pads and adipocytes (8, 10). Numerous intracellular enzymes have been reported to be both insulin and calcium sensitive (15)(16)(17)(18) (UDP glucose:glycogen 4-a-glucosyltransferase, EC 2.4.1.11) (18) isolated from adipocytes. Thus, small changes of calcium concentration at critical intracellular loci could have significant metabolic impact.The evaluation of cellular calcium distribution and regulation is difficult because intracellular calcium is highly compartmentalized with estimated concent...

Insulin‐induced Enhancement of Uptake of Noradrenaline in Atrial Strips

“…However, in the present study we failed to detect any change in the calcium accumulating abilities of heart sarcolemma, microsomes and mitochondria. It should be mentioned that some investigators (McDonald, Burns & Jarett, 1976) have shown a stimulatory effect of insulin on the nonspecific calcium binding by adipocyte plasma membranes while others have reported a depressant action by employing liver plasma membranes. Whether this discrepancy is due to tissue specificity or some other reason is not clear at this time.…”

Specific Binding Sites for Triiodothyronine in the Plasma Membrane of Rat Thymocytes

“…Progressive (31). With respect to the inhibitory effect of calcium, it is well known that plasma membranes have specific binding sites for calcium (32)(33)(34) and that calcium increases the binding of particular ligands to the plasma membranes of several tissues (35,36), but it is not known whether the binding of calcium initiates the latter responses.…”

Differences in organizational structure of insulin receptor on rat adipocyte and liver plasma membranes: role of disulfide bonds.

“…Adipocytes were isolated from rat epididymal fat pads as described (11) and plasma membranes were prepared as described by McKeel (12,13). The plasma membrane fractions were routinely frozen in dry ice/ethanol, stored at -70'C, and used within 1 week after preparation.…”