Differential sensitivity of insulin- and adaptive-regulation-induced system A activation to microtubular function in skeletal muscle

Abstract

4071. Insulin and adaptive regulation are known to stimulate system A amino acid transport activity in skeletal muscle. The present study was designed to investigate whether activation of system A in muscle is a consequence of processes which rely on microtubule or microfilament function. To that end, extensor digitorum longus (EDL) muscles were incubated in the presence of colchicine and cytochalasin D, well-known inhibitors of microtubule and microfilament activity respectively. 2. Basal a-(methyl)aminoisobutyric acid (MeAIB) uptake decreased after incubation with 5 jtM-colchicine in a time-dependent manner. In keeping with this, adaptive regulation of MeAIB uptake caused by prolonged incubation in the absence of amino acids was substantially decreased in the presence of colchicine. 3. Under these conditions, stimulation of MeAIB uptake by insulin was unaltered in muscle in the presence of colchicine. This contrasted with the insulin-induced stimulation of MeAIB uptake by isolated rat hepatocytes, which was markedly decreased by colchicine. 4. Cytochalasin D, an agent that disrupts microfilaments, did not inhibit basal or insulin-stimulated MeAIB uptake by the incubated muscle. 5. Neither colchicine nor cytochalasin D modified the stimulatory effect of insulin on 3-0-methylglucose uptake by EDL muscle. 6. We conclude that up-regulation of system A by synthesis of new carriers depends on the integrity of microtubular function both in skeletal muscle and in hepatocytes. Microtubules might play a role in the movement of system A-containing vesicles from the Golgi network to the plasma membrane.

Regulation of L-alanine transport systems A and ASC by cyclic AMP and calcium in a reptilian duodenal model

“…Moreover, the magnitude of system ASC activity stimulation was sufficient to explain the increase in overall Lalanine transport observed following forskolin. Consistent with our results, studies performed in skeletal muscle and placental membranes have demonstrated that neither forskolin nor cholera toxin, both cAMP-elevating factors, affect system A activity (Guma et al, 1992;Pastor-Anglada et al, 1996). Likewise, results from trout hepatocytes have revealed that system ASC, but not system A, was rapidly stimulated by glucagon (Gallardo et al, 1996).…”

Regulatory and molecular aspects of mammalian amino acid transport

“…An interesting feature of insulin action is the evidence that most physiological situations associated with System A up-regulation in liver parenchymal cells are characterized either by insulin deficiency or by resistance to the hormone. Some examples are: diabetes [41][42][43] where skeletal muscle also shows increased System A activity [44], starvation [45][46][47], pregnancy [48,49], and post-natal development [50][51][52]. Insulin also down-regulates the liver System A activity of streptozotocin-induced diabetic rats, either when added to cells in culture or after administration in vivo [41,42].…”

Osmoregulation of Neutral Amino Acid Transport

“…System A upregulation in many cells required intact microtubules. For example, upregulation of system A transport in skeletal muscle in response to insulin or adaptation involves synthesis of new carriers and requires the integrity of microtubule function (53). Colchicine has also been found to prevent the increased capacity for system A transport in the proliferating hepatocytes during liver regeneration (54).…”