Effects of excitation and inactivation in area 17 on paired cells in area 18.

This investigation examines how neighboring neurons of area 18 react when area 17 inputs are excited or depressed. In anesthetized cats, area 18 responses to a sine-wave grating in the receptive field were analyzed, while a second grating was positioned in its periphery and responses were recorded in area 17. This latter site was also inactivated with GABA. A waveform template process sorted out at least two individual, neighboring cells with similar orientation preferences in area 18. These cells frequently displayed opposite reactions to stimulation and inactivation in area 17. Experiments suggest that nearby neurons belonging to the same functional domain in the visual cortex may simultaneously carry disparate information.

Nitric oxide and its influence on oscillations of collicular responses in developing rats.

We investigated the influence of modulating NO synthesis on oscillatory components of ON and OFF evoked field potentials in developing rat superior colliculus. Nitric oxide (NO) is involved in neuronal transmission by adjusting neurotransmitter release in adults and in stabilizing synaptic connections in developing brains. NO synthesis was decreased by inhibiting nitric oxide synthase (NOS) with an acute microinjection of N-nitro-L-arginine methyl ester (L-NAME); whereas NO synthesis was augmented by an acute microinjection of L-arginine (L-ARG). The study is focused on rhythmic activity by analyzing fast Fourier transform (FFT). Collicular responses were recorded in anesthetized rats, at post-natal days (PND) 13-19 and adults. This time window was chosen because it is centered on eye opening. NO down- and upregulation resulted in a dual effect depending on age and response-type. NO synthesis inhibition decreased the magnitude of oscillations in ON responses in the youngest animals (PND13-PND14), whereas oscillations of frequencies higher than 20 Hz in OFF responses were increased in all age groups of developing rats. In adults NO downregulation increased oscillations in ON responses and decreased oscillations in OFF responses. L-arginine application produced effects opposite to those seen with L-NAME. Our data together with results reported in the literature suggest that the temporal patterns of the evoked activity are NO-dependent. This sculpting action of the evoked firing may play a role in the synchronization of action potentials in afferent axons which in turn contributes to synaptic stabilization.

Platelet protein kinase C isoform content in type 2 diabetes complicated with retinopathy and nephropathy.

It has been reported that platelet aggregation in diabetic patients with microangiopathy is increased compared with healthy subjects. Chronic hyperglycemia is known to cause an increase in diacylglycerol level in various tissues. We examine whether protein kinase C (PKC) isoform content in platelets from diabetic patients is increased compared with healthy subjects, as previously described in the retina, aorta, and heart of diabetic rats. Platelet PKCalpha, beta and zeta immunoreactivity in cytosol, membrane and cytoskeleton (CS) fractions were analyzed by immunoblotting in 20 type 2 diabetic patients (who had been treated with diet alone, sulphonylureas or insulin, and whose condition was complicated with retinopathy, nephropathy, neuropathy and/or macroangiopathy) and in five healthy subjects. PKCalpha, beta and zeta immunoreactivity in cytosol, membrane and CS fractions in platelets from diabetic subjects were not significantly higher than those from healthy subjects. However, platelet PKCbeta immunoreactivity in cytosol fraction was significantly higher in diabetic patients with normal serum creatinine (Cr) level than in diabetic patients with abnormal Cr level (Cr > or =1.5 mg/dl) or in healthy subjects. Moreover, significant negative correlation between PKCbeta immunoreactivity in cytosol fraction of platelets and serum Cr level was found in diabetic patients (P < 0.05). To clarify the effect of treatment for diabetes, PKC isoform immunoreactivity in platelets was measured in type 2 diabetic patients treated with diet alone, sulphonylurea or insulin treatment. Serum creatinine level in diabetic patients with insulin treatment was significantly higher than in diabetic patients with sulphonylurea treatment and diet alone. In addition, PKCbeta immunoreactivity in diabetic patients with insulin treatment was significantly suppressed compared with that in patients treated by sulphonylurea treatment. These results suggest that chronic hyperglycemia may activate platelet PKCbeta isoform, and that insulin treatment may decrease platelet PKCbeta activity. Finally, not only PKCbeta antagonists, but also glycemic control by insulin may prevent development of diabetic microangiopathy.

Effect of tumor necrosis factor-alpha on insulin signal transduction in rat adipocytes: relation to PKCbeta and zeta translocation.

Although much evidence has been accumulated suggesting that tumor necrosis factor-alpha (TNF-alpha) is an important mediator of insulin resistance, the precise mechanism involved is still unclear. Recently, it has been reported that insulin-induced glucose uptake is mediated by activation of second messengers such as insulin receptor substrate 1 (IRS-1), phosphatidylinositol 3-kinase (PI3K), and diacylglycerol (DG)-protein kinase C (PKC). We have examined the effect of TNF-alpha on insulin-induced glucose uptake and activations of tyrosine kinase, IRS-1, PI3K and PKC in rat adipocytes. Pretreatment with 0.1-100 nM TNF-alpha for 60 min resulted in a significant decrease in 10 nM insulin- or 1 microM 12-O-tetradecanoyl phorbol-13-acetate (TPA)-induced [3H]2-deoxyglucose uptake without affecting basal glucose uptake. 10 nM insulin-stimulated activation of tyrosine kinase, IRS-1 and PI3K was suppressed by preincubation with 0.1-10 nM TNF-alpha for 60 min. 10 nM TNF-alpha pretreatment also suppressed 10 nM insulin- and 1 microM TPA-induced increases in membrane-associated PKCbeta and PKCzeta. Furthermore, 10 nM TNF-alpha, by itself, altered PKCbeta translocation from the membrane to cytosol. These results suggest that TNF-alpha inhibits insulin-stimulated activation of both the tyrosine kinase-IRS-1-PI3K-PKCzeta pathway and DG-PKC pathway. Finally, TNF-alpha contributes to insulin resistance in rat adipocytes.

Maturation of visual receptive field properties in the rat superior colliculus.

Visually responsive neurons were recorded in the superficial layers of rat superior colliculus from postnatal day 12 to 28. Receptive field properties such as size, type (ON, OFF, ON-OFF and motion sensitive) and direction selectivity were analyzed to disclose changes during maturation. Although some aspects of sensory properties are modified during development (latency, receptive field sizes, and proportions of receptive field types), a high level of sophistication is also present in young animals even before eyelid opening. For instance, direction selective and direction biased cells, which require complex synaptic relations, are already observed when the first light evoked responses emerge in the superior colliculus (P13), strongly suggesting that this property develops without visual experience. Furthermore, direction selectivity is present in the colliculus prior to the appearance of visually evoked activity in the cortex. This indicates that direction selectivity can not be attributable to incoming cortical afferents. This study provides the first direct evidence that, unlike the cat, the rat's cortico-tectal pathway is only weakly involved in the establishment of direction selectivity in collicular neurons.

DHEA improves glucose uptake via activations of protein kinase C and phosphatidylinositol 3-kinase.

We have examined the effect of adrenal androgen, dehydroepiandrosterone (DHEA), on glucose uptake, phosphatidylinositol (PI) 3-kinase, and protein kinase C (PKC) activity in rat adipocytes. DHEA (1 microM) provoked a twofold increase in 2-[3H]deoxyglucose (DG) uptake for 30 min. Pretreatment with DHEA increased insulin-induced 2-[3H]DG uptake without alterations of insulin specific binding and autophosphorylation of insulin receptor. DHEA also stimulated PI 3-kinase activity. [3H]DHEA bound to purified PKC containing PKC-alpha, -beta, and -gamma. DHEA provoked the translocation of PKC-beta and -zeta from the cytosol to the membrane in rat adipocytes. These results suggest that DHEA stimulates both PI 3-kinase and PKCs and subsequently stimulates glucose uptake. Moreover, to clarify the in vivo effect of DHEA on Goto-Kakizaki (GK) and Otsuka Long-Evans fatty (OLETF) rats, animal models of non-insulin-dependent diabetes mellitus (NIDDM) were treated with 0.4% DHEA for 2 wk. Insulin- and 12-O-tetradecanoyl phorbol-13-acetate-induced 2-[3H]DG uptakes of adipocytes were significantly increased, but there was no significant increase in the soleus muscles in DHEA-treated GK/Wistar or OLETF/Long-Evans Tokushima (LETO) rats when compared with untreated GK/Wistar or OLETF/LETO rats. These results indicate that in vivo DHEA treatment can result in increased insulin-induced glucose uptake in two different NIDDM rat models.

Inhibitory effect of glyburide on thrombin-induced platelet aggregation and phosphoinositide metabolism in normal human platelets.

We previously reported the effects of diet, sulphonylureas or insulin on thrombin-induced platelet aggregation, phosphoinositide metabolism and protein phosphorylation in non-insulin-dependent diabetes mellitus (NIDDM) patients. To clarify the mechanism of glyburide and insulin on platelet function, here we studied the in vitro effects of glyburide and insulin on thrombin-induced metabolic changes using normal human platelets. Platelet aggregation stimulated with <0.5 U/ml thrombin, 0.75-3 microM adenosine diphosphate (ADP) or 1 microg/ml collagen was significantly lower in glyburide-treated platelets, but not in insulin-treated platelets, than in untreated ones (control). Thrombin-induced incorporation of 32P radioactivity into phosphatidic acid (PA) in glyburide-treated platelets was lower than that in control but not in insulin-treated platelets. Phosphorylated proteins of platelets induced by thrombin and 12- O -tetradecanoylphorbol 13-acetate (TPA) in glyburide-treated platelets were suppressed, but not in insulin-treated platelets, compared with control. These results suggest that glyburide induces suppression of thrombin-induced activation of phospholipase C, which mediates hydrolysis of PIP and PIP(2) and production of PA, and subsequently inhibits platelet aggregation.

Glucose- and phorbol ester-induced insulin secretion in human insulinoma cells--association with protein kinase C activation.

This study examined the effect of glucose and 12-O-tetradecanoylphorbol-13-acetate (TPA) on insulin secretion in isolated human insulinoma cells. In addition, we analyzed conventional PKC alpha and beta activation in the membrane fractions, respectively. Treatment with 5 mM and 20 mM glucose for 5 min and 20 min resulted in 6-7-fold increases in insulin secretion, and treatment with 1 microM TPA for 5 min also resulted in 3-fold increases in insulin secretion from the basal level. Immunoblot analysis of membrane fractions showed increases in PKC alpha and beta immunoreactivities after treatment with 5 mM, 20 mM glucose and 1 microM TPA. Translocations of PKC alpha after treatment with glucose and TPA were greater than those of PKC beta in membrane fractions. These results suggest that TPA independently provokes insulin secretion via PKC activation and that PKC alpha and beta activation may be involved in insulin secretion in human insulinoma cells.

Differential effect of the antidiabetic thiazolidinediones troglitazone and pioglitazone on human platelet aggregation mechanism.

Troglitazone and pioglitazone, antidiabetic thiazolidinediones, are known to improve insulin resistance. However, the effect of these drugs on platelet aggregation remains unclear. The chemical structure of troglitazone contains vitamin E. Accordingly, we studied the effect of troglitazone, pioglitazone, and vitamin E on thrombin-induced platelet aggregation, metabolism of phosphoinositide, protein phosphorylation, protein kinase C (PKC)-alpha and -beta, and phosphatidylinositol (PI) 3-kinase activation in vitro in human platelets. Maximum platelet aggregation by ADP, collagen, and thrombin decreased in the presence of 0.1-1 micromol/l troglitazone and 500 nmol/l vitamin E for 60 min compared with controls. However, pioglitazone did not inhibit ADP-, collagen-, or thrombin-induced platelet aggregation. Pretreatment with troglitazone and vitamin E, but not with pioglitazone, resulted in decreases in thrombin-induced phosphatidic acid production, hydrolysis of phosphatidylinositol 4,5-bisphosphate by phospholipase C, and 47-kDa protein phosphorylation. Thrombin-induced PKC-alpha and -beta activation in membrane fraction was suppressed by pretreatment with troglitazone and vitamin E, but not with pioglitazone. Separately, troglitazone and pioglitazone stimulated PI 3-kinase activity, but thrombin-induced PI 3-kinase activation was suppressed by pretreatment with troglitazone and pioglitazone for 60 min. These results suggest that troglitazone and vitamin E, but not pioglitazone, have a potent inhibitory effect on platelet aggregation via suppression of the thrombin-induced activation of phosphoinositide signaling in human platelets. Finally, the chemical structure of vitamin E may contribute to the inhibitory effect of troglitazone on platelet aggregation in human platelets.

Alterations in insulin-induced postreceptor signaling in adipocytes of the Otsuka Long-Evans Tokushima fatty rat strain.

The Otsuka Long-Evans Tokushima fatty (OLETF) rat is a new spontaneous non-insulin-dependent diabetes mellitus (NIDDM) model rat strain developed in Tokushima, Japan. After 18 weeks of age, decreases of 45% and 40% respectively in insulin- and phorbol ester-stimulated [3H]2-deoxyglucose (DOG) uptake were observed, compared with those in Long-Evans Tokushima (LETO) rats (control). Insulin-specific binding and 95 kDa autophosphorylation of insulin receptor in OLETF rats were not different from those in LETO rats. Insulin-induced diacylglycerol (DG) production and Mono Q column-purified protein kinase C (PKC) translocation in adipocytes of OLETF rats were decreased compared with those of LETO rats. Insulin-induced PKC beta translocation from cytosol to membrane was also decreased in adipocytes of OLETF rats. Increases of the PKC beta I, beta II, epsilon and zeta isoforms in membranes of OLETF rats were markedly smaller than those of LETO rats. Analysis of mRNA levels of PKC isoforms in adipocytes of OLETF rats showed decreases of basal level and insulin-induced delayed responses of PKC beta I, beta II, epsilon and zeta mRNA in OLETF rats. On the other hand, insulin- or phorbol ester-induced phosphatidylinositol 3-kinase (PI 3-kinase) activation was decreased in adipocytes of OLETF rats compared with those of LETO rats. These results suggest that insulin resistance in OLETF rats, a spontaneous NIDDM model rat, may be associated with deterioration of insulin-induced DG-PKC signaling and subsequent decrease in PI 3-kinase activation.

Effect of glucocorticoid receptor antagonist RU 38486 on acute glucocorticoid-induced insulin resistance in rat adipocytes.

We examined the mechanism of acute glucocorticoid-induced insulin resistance in rat adipocytes using the glucocorticoid receptor antagonist RU 38486. Pretreatment with dexamethasone (DEX) and prednisolone for 60 minutes resulted in 50% inhibition of insulin-induced [3H]2-deoxyglucose (DOG) uptake at 10(-8) and 10(-7) mol/L, respectively, in rat adipocytes and 20% and 25% inhibition of insulin-induced [3H]2-DOG uptake, respectively, in soleus muscles. Our previous experiments indicated that DEX and prednisolone alone stimulate protein kinase C (PKC) in rat adipocytes. Accordingly, we examined [3H]DEX binding to PKC from MonoQ column-purified rat brain cytosol. Specific [3H]DEX binding to MonoQ column-purified PKC was observed (kd, 56.8 nmol/L; Bmax, 725 fmol/mg protein). Thus, insulin-induced PKC translocation from the cytosol to the membrane was suppressed by pretreatment with 10(-7) mol/L DEX and 10(-6) mol/L prednisolone for 80 minutes. During treatment with RU 38486 for 60 minutes, there was no change in the glucocorticoid-induced inhibitory effect on insulin-induced [3H]2-DOG uptake and PKC translocation from the cytosol to the membrane. Moreover, pretreatment with RU 38486 for 120 minutes slightly prevented the DEX-mediated inhibition of insulin-induced glucose uptake. These results suggest that acute glucocorticoid-induced insulin resistance may be mainly mediated through the other non-glucocorticoid receptor pathway.

Acute effects of phorbol ester and insulin on insulin-induced glucose uptake and protein kinase C activation in rat adipocytes.

We examined the acute effect of pretreatment with phorbol ester and insulin on insulin-induced glucose uptake and protein kinase C (PKC) translocation from cytosol to the membrane in rat adipocytes. Adipocytes were preincubated with 1 microM tetradecanoylphorbol 13-acetate (TPA) and 10 nM insulin for 60 min and then stimulated with 10 nM insulin for 10 and 30 min to measure PKC activity in cytosol and membrane fractions using a Mono Q column connected onto an HPLC system and [3H]2-deoxyglucose (DOG) uptake, respectively. Pretreatment with 1 microM TPA and 10 nM insulin for 60 min resulted in the marked decreases of insulin-induced [3H]2-DOG uptake. Translocation of Mono Q column-purified cytosolic PKC enzyme activity and PKC beta immunoreactivity from cytosol to the membrane was suppressed by pretreatment with TPA and insulin for 60 min. These results indicate that acute treatment with TPA and insulin which are PKC activators suppress translocation/activation of PKC, and accordingly inhibit insulin-induced glucose uptake. We suggest that a decrease of cytosolic PKC activity may mainly-contribute to the impaired responsiveness of the glucose transport system after acute TPA and insulin treatment.

ON and OFF field potentials in the rat superior colliculus during development.

The present investigation is aimed at characterizing the development of ON and OFF visually evoked responses in the rat superior colliculus from postnatal day 13 (P13) to postnatal day 25. Depth profiles of field potentials reveal that ON and OFF long latency biphasic field potentials are already present when collicular cells are first responsive to light (P13). There is an inversion in the polarity of these responses as the electrode penetrates the collicular layers, suggesting a synaptic organization similar to the one found in adult animals. At P15, OFF field potentials begin to exhibit oscillatory activity. Local cobalt injections within the superior colliculus abolishes these OFF oscillations, suggesting a postsynaptic origin. Fast Fourier transform (FFT) analysis of the OFF field potentials demonstrates that oscillatory activity increases in frequency during development. This increase is thought to reflect the myelination and stabilization of synaptic connections that occur during this period. To our knowledge, this is the first report of OFF oscillatory responses in the superior colliculus.

ACTH-induced cortisol secretion is mediated by cAMP and PKC in various adrenocortical adenomas.

We examined the role of PKC in cortisol secretion from adrenocortical adenomas. Isolated cells were prepared from aldosterone producing adenoma (APA, n=5), APA complicated with pre-clinical Cushing's syndrome (APA+PC, n=1), PC (n=2), and cortisol producing adenoma (CPA, n=5). They were stimulated with 100 nM ACTH, 1 microM forskolin (FS), 1 microM tetradecanoyl phorbol 13-acetate (TPA), and 100 nM angiotensin II (AngII). ACTH was most potent to secret cortisol. FS also stimulated cortisol secretion, but did less-potently. TPA and AngII also stimulated cortisol secretion significantly in cells from CPA. Furthermore, ACTH- and TPA-induced PKCalpha and beta translocations from cytosol to membrane were observed in adenoma cells from APA+PC, PC, and CPA. In conclusion, it was suggested that ACTH-induced cortisol secretion may be mediated by both PKC and protein kinase A in adrenocortical adenomas, and that PKC-mediated signal transduction may be involved in ACTH-induced cortisol secretion in CPA.

Insulin regulated PKC isoform mRNA in rat adipocytes.

Insulin and 12-O-tetradecanoyl phorbol-13-acetate (TPA) induce both glucose uptake and translocation of protein kinase C (PKC) from cytosol to membrane in insulin-sensitive tissues as previously reported by several investigators. We examined insulin-mediated PKC beta I, beta II, and epsilon translocation from cytosol to cytoskeleton, and expression of PKC alpha, beta I, beta II, gamma, and epsilon isoforms using the reverse transcription polymerase chain reaction (RT-PCR) method during treatment with insulin for 240 min in rat adipocytes. Insulin-induced increases in PKC beta I, beta II, and epsilon were greater in the cytoskeleton fraction than those in the membrane fraction. Insulin induced time-dependent increases in PKC alpha, gamma, epsilon and zeta mRNA levels for up to 240 min (555%, 117%, 236% and 138% increase, respectively). TPA also induced time-dependent increases in PKC alpha and gamma (34% and 500% increase, respectively) but not in PKC zeta. However, PKC beta I mRNA was decreased for up to 60 min and then maintained at under the basal level during stimulation with insulin and TPA. On the other hand, PKC beta II mRNA was markedly increased for up to 240 min. These results suggest that insulin-regulated PKC alpha, gamma and epsilon mRNA levels and PKC beta mRNA alternative splicing may occur in rat adipocytes.

Effects of wortmannin on glucose uptake and protein kinase C activity in rat adipocytes.

Wortmannin is known to be an inhibitor of myosin light chain kinase and phosphatidylinositol 3-kinase (PI 3-kinase) (J. Biol. Chem. 268, 25846, 1993). We studied the effects of wortmannin on insulin- and 12-O-tetradecanoylphorbol 13-acetate (TPA)-induced glucose uptake, purified PKC activity and in vitro 80 kDa protein phosphorylation to elucidate the relationship between insulin-induced PI 3-kinase and PKC activations. Pretreatment with 10(-12)-10(-6) M wortmannin for 60 min resulted in a dose-responsive reduction of 10 nM insulin-stimulated glucose uptake in rat adipocytes. Pretreatment with 10(-6) M wortmannin resulted in 80% and 20% decreases of glucose uptake stimulated by insulin and TPA, respectively. Partially purified rat brain PKC activity and 80 kDa protein in vitro phosphorylation of rat adipocyte cytosol by addition of Ca2+ and phospholipid were dose-dependently decreased by 10(-8)-10(-6) M wortmannin; 20% decrease of PKC activity and 50% decrease of 80 kDa protein phosphorylation by 10(-6) M wortmannin were observed. These results suggest that wortmannin has a potent inhibitory effect on PI 3-kinase and a weak inhibitory effect on PKC activity, and both effects cause a significant inhibition of insulin-stimulated glucose uptake in rat adipocytes.

Evolution of spontaneous activity in the developing rat superior colliculus.

During the first 10 days after birth in the rat there are a succession of major developmental stages in the retinotectal pathway. During most of this time, the only recordable event in the superior colliculus is spontaneous activity. We studied and characterized this spontaneous activity, hypothesizing that it could play an important role in pathway development. The spontaneous discharges are detectable on postnatal day 5 (P5). After P5, the number of spontaneously active cells per penetration increases up to P10, after which they decrease to adult-like levels by P14-P15. Between P5 and P10, the spontaneous discharges exhibit several patterns of activity, from constant firing to intermittent bursts with periods of quiescence, without any bearing to age. We isolated the retina and superior colliculus by injecting xylocaine onto the optic nerve and found no change in collicular activity. While this suggests that the spontaneous activity in the colliculus is independent of the retina at the ages studied, the opposite experiment, i.e., electrically stimulating the optic nerve, resulted in increased firing by collicular neurons, perhaps via nonclassical synaptic transmission. Finally, we compared interval histograms for spontaneously active cells between P5 and P15. The histograms suggest that at certain ages, spontaneous firing is more regular; moreover, these ages precede major functional advances, e.g., onset of numerous spontaneously firing cells at P6, the first response to optic nerve stimulation at P10, and the first light-evoked response at P12-P13. Our results support the hypothesis that spontaneous activity in the neonatal superior colliculus has a role in development of the retinotectal pathway, but the data also indicate that classical synaptic transmission is not involved.

Renal refractoriness to phosphaturic action of parathyroid hormone in a patient with hypomagnesemia.

A 50-year-old tetanic woman with hypomagnesemia is described. She had partial resection of the stomach and the jejunum at the age of 20 years. Lack of parathyroid hormone (PTH) function was indicated by hypocalcemia, hyperphosphatemia and high tubular reabsorption of phosphate. However, both plasma concentration of PTH and nephrogenous cAMP were normal. Administration of magnesium sulfate completely normalized serum phosphate and tubular transport of phosphate with only a modest increase in nephrogenous cAMP. The present findings suggest that phosphaturic action of PTH is impaired in magnesium deficiency and that steps distal to cAMP production may be responsible for the renal refractoriness to the hormonal action.

Phorbol ester and insulin stimulate protein kinase C isoforms in rat adipocytes.

We examined effect of insulin or 12-O-tetradecanoyl phorbol 13-acetate (TPA) on the subcellular redistribution of protein kinase C isoforms in rat adipocytes. Total Mono Q column-elutable novel PKCs (nPKCs) which are Ca(2+)-independent and phospholipid-dependent protein kinases, decreased in the cytosolic fraction and increased in the membrane fraction during treatment with insulin or phorbol ester for 10 min. Immunoblot analysis of novel PKCs, -epsilon, -delta and -zeta, showed that insulin stimulated the translocation of these PKC isoforms from cytosol to membrane, similar to the translocation of conventional Ca2+/phospholipid-dependent PKCs (cPKCs), -alpha, -beta, and -gamma. Phorbol esters stimulated the translocation of PKC-alpha, -beta, -gamma, -epsilon and -delta, but not PKC-zeta. These results suggest that (a) insulin and phorbol esters similarly stimulate the translocation of each PKC isoform except for PKC-zeta, and (b) the translocation of both nPKCs and cPKCs occurs during insulin and TPA actions in rat adipocytes.