Segmental heterogeneity in the biochemical properties of the Na+-K+-ATPase along the intestine of the gilthead seabream (Sparus aurata L.)

The activity of the Na+-K+-ATPase along the intestinal mucosa of the gilthead seabream has been examined. Under optimal assay conditions, found at 35 degrees C, pH 7.5, 2-5 mM MgCl2, 5 mM ATP, 10 mM K+ and 200 mM Na+, maximal Na+-K+-ATPase activities were found in the microsomal fraction of pyloric caeca (PC) and anterior intestine (AI), which were more than two-fold the activity measured in the microsomes from the posterior intestine (PI). Na+-K+-ATPase activities from PC, AI and PI displayed similar pH dependence, optimal Mg2+/ATP and Na+/K+ ratios, affinities for Mg2+ and ATP, and inhibition by vanadate. However, considerable differences regarding sensitivity to ouabain, inhibition by calcium and responses to ionic strength were observed between segments. Thus, Na+-K+-ATPase activity from the AI was found to be ten-fold more sensitive to ouabain and calcium than the enzyme from the PC and PI and displayed distinct kinetic behaviours with respect to Na+ and K+, compared to PC and PI. Analysis of the data from the AI revealed the presence of two Na+-K+-ATPase activities endowed with distinguishable biochemical characteristics, suggesting the involvement of two different isozymes. Regional differences in Na+-K+-ATPase activities in the intestine of the gilthead seabream are compared with literature data on Na+-K+-ATPase isozymes and discussed on the basis of the physiological differences between intestinal regions.     

Parathyroid hormone-mediated regulation of Na+-K+-ATPase requires ERK-dependent translocation of protein kinase Calpha

Parathyroid hormone (PTH) inhibits Na+-K+-ATPase activity by serine phosphorylation of the alpha1 subunit through protein kinase C (PKC)- and extracellular signal-regulated kinase (ERK)-dependent pathways. Based on previous studies we postulated that PTH regulates sodium pump activity through isoform-specific PKC-dependent activation of ERK. In the present work utilizing opossum kidney cells, a model of renal proximal tubule, PTH stimulated membrane translocation of PKCalpha by 102 +/- 16% and PKCbetaI by 41 +/- 7% but had no effect on PKCbetaII and PKCzeta. Both PKCalpha and PKCbetaI phosphorylated the Na+-K+-ATPase alpha1 subunit in vitro. PTH increased the activity of PKCalpha but not PKCbetaI. Coimmunoprecipitation assays demonstrated that treatment with PTH enhanced the association between Na+-K+-ATPase alpha1 subunit and PKCalpha, whereas the association between Na+-K+-ATPase alpha1 subunit and PKCbetaI remained unchanged. A PKCalpha inhibitory peptide blocked PTH-stimulated serine phosphorylation of the Na+-K+-ATPase alpha1 subunit and inhibition of Na+-K+-ATPase activity. Pharmacologic inhibition of MEK-1 blocked PTH-stimulated translocation of PKCalpha, whereas transfection of constitutively active MEK-1 cDNA induced translocation of PKCalpha and increased phosphorylation of the Na+-K+-ATPase alpha1 subunit. In contrast, PTH-stimulated ERK activation was not inhibited by pretreatment with the PKCalpha inhibitory peptide. Inhibition of PKCalpha expression by siRNA did not inhibit PTH-mediated ERK activation but significantly reduced PTH-mediated phosphorylation of the Na+-K+-ATPase alpha1 subunit. Pharmacologic inhibition of phosphoinositide 3-kinase blocked PTH-stimulated ERK activation, translocation of PKCalpha, and phosphorylation of the Na+-K+-ATPase alpha1 subunit. We conclude that PTH stimulates Na+-K+-ATPase phosphorylation and decreases the activity of Na+-K+-ATPase by ERK-dependent activation of PKCalpha.     

Hyperglycemia increases endothelial superoxide that impairs smooth muscle cell Na+-K+-ATPase activity

Nitric oxide (NO) plays an important role in the control of numerous vascular functions including basal Na+-K+-ATPase activity in arterial tissue. Hyperglycemia inhibits Na+-K+-ATPase activity in rabbit aorta, in part, through diminished bioactivity of NO. The precise mechanism(s) for such observations, however, are not yet clear. The purpose of this study was to examine the role of superoxide in modulating NO-mediated control of Na+-K+-ATPase in response to hyperglycemia. Rabbit aorta incubated with hyperglycemic glucose concentrations (44 mM) demonstrated a 50% reduction in Na+-K+-ATPase activity that was abrogated by superoxide dismutase. Hyperglycemia also produced a 50% increase in steady-state vascular superoxide measured by lucigenin-enhanced chemiluminescence that was closely associated with reduced Na+-K+-ATPase activity. Specifically, the hyperglycemia-induced increase in vascular superoxide was endothelium dependent, inhibited by L-arginine, and stimulated by N(omega)-nitro-L-arginine. Aldose reductase inhibition with zopolrestat also inhibited the hyperglycemia-induced increase in vascular superoxide. In each manipulation of vascular superoxide, a reciprocal change in Na+-K+-ATPase activity was observed. Finally, a commercially available preparation of Na+-K+-ATPase was inhibited by pyrogallol, a superoxide generator. These data suggest that hyperglycemia induces an increase in endothelial superoxide that inhibits the stimulatory effect of NO on vascular Na+-K+-ATPase activity.     

Gill Na+-K+-ATPase activity correlates with basolateral membrane lipid composition in seawater- but not freshwater-acclimated Arctic char (Salvelinus alpinus)

The successful migration of euryhaline teleost fish from freshwater to seawater requires the upregulation of gill Na+-K+-ATPase, an ion transport enzyme located in the basolateral membrane (BLM) of gill chloride cells. Following 39 days of seawater exposure, Arctic char had similar plasma sodium and chloride levels as individuals maintained in freshwater, indicating they had successfully acclimated to seawater. This acclimation was associated with an eightfold increase in gill Na+-K+-ATPase activity but only a threefold increase in gill Na+-K+-ATPase protein number, suggesting that other mechanisms may also modulate gill Na+-K+-ATPase activity. We therefore investigated the influence of membrane composition on Na+-K+-ATPase activity by examining the phospholipid, fatty acid, and cholesterol composition of the gill BLM from freshwater- and seawater-acclimated Arctic char. Mean gill BLM cholesterol content was significantly lower ( approximately 22%) in seawater-acclimated char. Gill Na+-K+-ATPase activity in individual seawater Arctic char was negatively correlated with BLM cholesterol content and positively correlated with %phosphatidylethanolamine and overall %18:2n6 (linoleic acid) content of the BLM, suggesting gill Na+-K+-ATPase activity of seawater-acclimated char may be modulated by the lipid composition of the BLM and may be especially sensitive to those parameters known to influence membrane fluidity. Na+-K+-ATPase activity of individual freshwater Arctic char was not correlated to any membrane lipid parameter measured, suggesting that different lipid-protein interactions may exist for char living in each environment.     

Phospholemman overexpression inhibits Na+-K+-ATPase in adult rat cardiac myocytes: relevance to decreased Na+ pump activity in postinfarction myocytes.

Messenger RNA levels of phospholemman (PLM), a member of the FXYD family of small single-span membrane proteins with putative ion-transport regulatory properties, were increased in postmyocardial infarction (MI) rat myocytes. We tested the hypothesis that the previously observed reduction in Na+-K+-ATPase activity in MI rat myocytes was due to PLM overexpression. In rat hearts harvested 3 and 7 days post-MI, PLM protein expression was increased by two- and fourfold, respectively. To simulate increased PLM expression post-MI, PLM was overexpressed in normal adult rat myocytes by adenovirus-mediated gene transfer. PLM overexpression did not affect the relative level of phosphorylation on serine68 of PLM. Na+-K+-ATPase activity was measured as ouabain-sensitive Na+-K+ pump current (Ip). Compared with control myocytes overexpressing green fluorescent protein alone, Ip measured in myocytes overexpressing PLM was significantly (P < 0.0001) lower at similar membrane voltages, pipette Na+ ([Na+]pip) and extracellular K+ ([K+]o) concentrations. From -70 to +60 mV, neither [Na+]pip nor [K+]o required to attain half-maximal Ip was significantly different between control and PLM myocytes. This phenotype of decreased V(max) without appreciable changes in K(m) for Na+ and K+ in PLM-overexpressed myocytes was similar to that observed in MI rat myocytes. Inhibition of Ip by PLM overexpression was not due to decreased Na+-K+-ATPase expression because there were no changes in either protein or messenger RNA levels of either alpha1- or alpha2-isoforms of Na+-K+-ATPase. In native rat cardiac myocytes, PLM coimmunoprecipitated with alpha-subunits of Na+-K+-ATPase. Inhibition of Na+-K+-ATPase by PLM overexpression, in addition to previously reported decrease in Na+-K+-ATPase expression, may explain altered V(max) but not K(m) of Na+-K+-ATPase in postinfarction rat myocytes.     

温度和盐度对吉富品系尼罗罗非鱼幼鱼鳃Na+-K+-ATPase活力的联合效应

试验采用中心组合设计(central composite face-centered design,CCF)和响应曲面法(response surface methodology,RSM)研究了温度(12—34℃)和盐度(0—26)两因素对体长为(4.36±0.105)cm,体重为(2.45±0.153)g的吉富品系尼罗罗非鱼(GIFT Nile tilapia,Oreochromis niloticus;简称吉富罗非鱼)幼鱼鳃Na+-K+-ATPase活力的联合效应。结果表明:(1)温度和盐度的一次效应和二次效应对Na+-K+-ATPase活力影响极显著(P<0.01),温度和盐度的互作效应不显著(P>0.05);(2)经响应曲面法分析,随着温度和盐度的增大,Na+-K+-ATPase活力呈先减小后增大的趋势;(3)建立了Na+-K+-ATPase活力与温度、盐度间关系的模型方程(R2=0.9829,Pred.R2=0.8550,P<0.01),并可用于预测吉富罗非鱼幼鱼鳃Na+-K+-ATPase的活力;(4)优化结果显示,温度为24.15℃,盐度为11.75时,Na+-K+-ATPase活力最小为0.62μmol无机磷.mg-1蛋白.h-1,满意度函数值高达0.961。Na+-K+-ATPase活力可以作为检测罗非鱼生长性能的指标,其活力较低时,一般反映了鱼体生存环境适宜,生长代谢旺盛,消耗于渗透调节的能量较少。     

Human nongastric H+-K+-ATPase: transport properties of ATP1al1 assembled with different beta-subunits

To investigate whether nongastric H+-K+-ATPases transport Na+ in exchange for K+ and whether different beta-isoforms influence their transport properties, we compared the functional properties of the catalytic subunit of human nongastric H+-K+-ATPase, ATP1al1 (AL1), and of the Na+-K+-ATPase alpha1-subunit (alpha1) expressed in Xenopus oocytes, with different beta-subunits. Our results show that betaHK and beta1-NK can produce functional AL1/beta complexes at the oocyte cell surface that, in contrast to alpha1/beta1 NK and alpha1/betaHK complexes, exhibit a similar apparent K+ affinity. Similar to Na+-K+-ATPase, AL1/beta complexes are able to decrease intracellular Na+ concentrations in Na+-loaded oocytes, and their K+ transport depends on intra- and extracellular Na+ concentrations. Finally, controlled trypsinolysis reveals that beta-isoforms influence the protease sensitivity of AL1 and alpha1 and that AL1/beta complexes, similar to the Na+-K+-ATPase, can undergo distinct K+-Na+- and ouabain-dependent conformational changes. These results provide new evidence that the human nongastric H+-K+-ATPase interacts with and transports Na+ in exchange for K+ and that beta-isoforms have a distinct effect on the overall structural integrity of AL1 but influence its transport properties less than those of the Na+-K+-ATPase alpha-subunit.     

Preconditioning attenuates ischemia-reperfusion-induced remodeling of Na+-K+-ATPase in hearts

The aim of this study was to determine whether changes in protein content and/or gene expression of Na+-K+-ATPase subunits underlie its decreased enzyme activity during ischemia and reperfusion. We measured protein and mRNA subunit levels in isolated rat hearts subjected to 30 min of ischemia and 30 min of reperfusion (I/R). The effect of ischemic preconditioning (IP), induced by three cycles of ischemia and reperfusion (10 min each), was also assessed on the molecular changes in Na+-K+-ATPase subunit composition due to I/R. I/R reduced the protein levels of the alpha2-, alpha3-, beta1-, and beta2-isoforms by 71%, 85%, 27%, and 65%, respectively, whereas the alpha1-isoform was decreased by <15%. A similar reduction in mRNA levels also occurred for the isoforms of Na+-K+-ATPase. IP attenuated the reduction in protein levels of Na+-K+-ATPase alpha2-, alpha3-, and beta2-isoforms induced by I/R, without affecting the alpha1- and beta1-isoforms. Furthermore, IP prevented the reduction in mRNA levels of Na+-K+-ATPase alpha2-, alpha3-, and beta1-isoforms following I/R. Similar alterations in protein contents and mRNA levels for the Na+/Ca2+ exchanger were seen due to I/R as well as IP. These findings indicate that remodeling of Na+-K+-ATPase may occur because of I/R injury, and this may partly explain the reduction in enzyme activity in ischemic heart disease. Furthermore, IP may produce beneficial effects by attenuating the remodeling of Na+-K+-ATPase and changes in Na+/Ca2+ exchanger in hearts after I/R.     

Seasonal changes in glycogen content and Na+-K+-ATPase activity in the brain of crucian carp

Changes in the number of Na+-K+-ATPase alpha-subunits, Na+-K+-ATPase activity and glycogen content of the crucian carp (Carassius carassius) brain were examined to elucidate relative roles of energy demand and supply in adaptation to seasonal anoxia. Fish were collected monthly around the year from the wild for immediate laboratory assays. Equilibrium dissociation constant and Hill coefficient of [3H]ouabain binding to brain homogenates were 12.87+/-2.86 nM and -1.18+/-0.07 in June and 11.93+/-2.81 nM and -1.17+/-0.06 in February (P>0.05), respectively, suggesting little changes in Na+-K+-ATPase alpha-subunit composition of the brain between summer and winter. The number of [3H]ouabain binding sites and Na-K-ATPase activity varied seasonally (P<0.001) but did not show clear connection to seasonal changes in oxygen content of the fish habitat. Six weeks' exposure of fish to anoxia in the laboratory did not affect Na+-K+-ATPase activity (P>0.05) confirming the anoxia resistance of the carp brain Na pump. Although anoxia did not suppress the Na pump, direct Q10 effect on Na+-K+-ATPase at low temperatures resulted in 10 times lower catalytic activity in winter than in summer. Brain glycogen content showed clear seasonal cycling with the peak value of 203.7+/-16.1 microM/g in February and a 15 times lower minimum (12.9+/-1.2) in July. In winter glycogen stores are 15 times larger and ATP requirements of Na+-K+-ATPase at least 10 times less than in summer. Accordingly, brain glycogen stores are sufficient to fuel brain function for about 8 min in summer and 16 h in winter, meaning about 150-fold extension of brain anoxia tolerance by seasonal changes in energy supply-demand ratio.     

Renal Na+-K+-ATPase in weanling and adult spontaneously hypertensive rats

The interrelationships among plasma renin activity (PRA, ng AI/ml plasma/hr), aldosterone concentration (ng%), and renal Na+-K+-ATPase activity (mumole PO4/mg protein/hr) were studied in 9 weanling normotensive spontaneously hypertensive rats (SHR), 9 adult hypertensive SHR, and 9 weanling and 9 adult normotensive Wistar-Kyoto rats (WKY). All groups were placed on a normal (0.4% sodium) diet. PRA and plasma aldosterone, measured in samples drawn from the ether-anesthetized rat, were higher in weanling SHR (15.2 +/- 2.0, 37 +/- 4.2) than in WKY. PRA measured in samples collected from a separate group of unanesthetized weanling SHR was also greater than in age-matched WKY. In adult SHR, PRA (6.1 +/- 0.9) and plasma aldosterone (20.0 +/- 2.7) were decreased. During the weanling period Na+-K+-ATPase activity in SHR was not only greater than in age-matched WKY but was also increased compared to adult normotensive and hypertensive rats (137 +/- 9 weanling SHR, 89 +/- 7 weanling WKY, 73 +/- 11 adult SHR, 84 +/- 17 adult WKY). Thus, during the weanling period the renin-angiotensin-aldosterone (R-A-A) system and renal Na+-K+-ATPase activity are activated in SHR. The elevation of Na+-K+-ATPase activity may be due to increased aldosterone levels. It was noted, however, that plasma aldosterone was similar in adult WKY and weanling SHR, while Na+-K+-ATPase activity was higher in SHR. These findings involving R-A-A and renal Na+-K+-ATPase activity prior to the elevation of blood pressure suggest that the kidneys may play a role in the initiation of hypertension in SHR.     

Ionic mechanisms of excitation-induced regulation of Na+-K+-ATPase mRNA expression in isolated rat EDL muscle

This study investigated the effects of electrical stimulation on Na+-K+-ATPase isoform mRNA, with the aim to identify factors modulating Na+-K+-ATPase mRNA in isolated rat extensor digitorum longus (EDL) muscle. Interventions designed to mimic exercise-induced increases in intracellular Na+ and Ca2+ contents and membrane depolarization were examined. Muscles were mounted on force transducers and stimulated with 60-Hz 10-s pulse trains producing tetanic contractions three times at 10-min intervals. Ouabain (1.0 mM, 120 min), veratridine (0.1 mM, 30 min), and monensin (0.1 mM, 30 min) were used to increase intracellular Na+ content. High extracellular K+ (13 mM, 60 min) and the Ca2+ ionophore A-23187 (0.02 mM, 30 min) were used to induce membrane depolarization and elevated intracellular Ca2+ content, respectively. Muscles were analyzed for Na+-K+-ATPase alpha1-alpha3 and beta1-beta3 mRNA (real-time RT-PCR). Electrical stimulation had no immediate effect on Na+-K+-ATPase mRNA; however at 3 h after stimulation, it increased alpha1, alpha2, and alpha3 mRNA by 223, 621, and 892%, respectively (P = 0.010), without changing beta mRNA. Ouabain, veratridine, and monensin increased intracellular Na+ content by 769, 724, and 598%, respectively (P = 0.001) but did not increase mRNA of any isoform. High intracellular K+ concentration elevated alpha1 mRNA by 160% (P = 0.021), whereas A-23187 elevated alpha3 mRNA by 123% (P = 0.035) but reduced beta1 mRNA by 76% (P = 0.001). In conclusion, electrical stimulation induced subunit-specific increases in Na+-K+-ATPase mRNA in isolated rat EDL muscle. Furthermore, Na+-K+-ATPase mRNA appears to be regulated by different stimuli, including cellular changes associated with membrane depolarization and increased intracellular Ca2+ content but not increased intracellular Na+ content.     

Participation of electrogenic Na+-K+-ATPase in the membrane potential of earthworm body wall muscles

The effect of Na+-K+-ATPase inhibitor ouabain on the resting membrane potential (Vm) was studied by glass microelectrodes in isolated somatic longitudinal muscles of the earthworm Lumbricus terrestris and compared with frog sartorius muscle. In earthworm muscle, Vm was -49 mV (inside negative) in a reference external solution with 4 mmol/l K+. The electrogenic participation of Na+-K+-ATPase was absent in solutions with very low concentrations of 0.01 mmol/l K+, higher in 4 and 8 mmol/l K+ (4-5 mV) and maximal (13 mV) in solutions containing 12 mmol/l K+ where Vm was -46 mV in the absence and -33 mV in the presence of 1 x 10(4) M ouabain. The electrogenic participation of Na+-K+-ATPase was much smaller in m. sartorius of the frog Rana temporaria bathed in 8 and 12 mmol/l K+. The results indicate that the Na+-K+-ATPase is an important electrogenic factor in earthworm longitudinal muscle fibres and that its contribution to Vm depends directly on the concentration of K+ in the bathing solution.     

Physiological role of the alpha1- and alpha2-isoforms of the Na+-K+-ATPase and biological significance of their cardiac glycoside binding site

An interesting feature of Na+-K+-ATPase is that it contains four isoforms of the catalytic alpha-subunit, each with a tissue-specific distribution. Our laboratory has used gene targeting to define the functional role of the alpha1- and alpha2-isoforms. While knockout mice demonstrated the importance of the alpha1- and alpha2-isoforms for survival, the knockin mice, in which each isoform can be individually inhibited by ouabain and its function determined, demonstrated that both isoforms are regulators of cardiac muscle contractility. Another intriguing aspect of the Na+-K+-ATPase is that it contains a binding site for cardiac glycosides, such as digoxin. Conservation of this site suggests that it may have an in vivo role and that a natural ligand must exist to interact with this site. In fact, cardiac glycoside-like compounds have been observed in mammals. Our recent study demonstrates that the cardiac glycoside binding site of the Na+-K+-ATPase plays a role in the regulation of blood pressure and that it mediates both ouabain-induced and ACTH-induced hypertension in mice. Whereas chronic administration of ouabain or ACTH caused hypertension in wild-type mice, it had no effect on blood pressure in mice with a ouabain-resistant alpha2-isoform of Na+-K+-ATPase. Interestingly, animals with the ouabain-sensitive alpha1-isoform and a ouabain-resistant alpha2-isoform develop ACTH-induced hypertension to a greater extent than wild-type animals. Taken together, these results demonstrate that the cardiac glycoside binding of the Na+-K+-ATPase has a physiological role and suggests a function for a naturally occurring ligand that is stimulated by administration of ACTH.     

Influence of monensin on cation influx and Na+ -K+ -ATPase activity of Eimeria tenella sporozoites in vitro

The experiments were conducted to determine intrasporozoite Na+/K+ concentrations (by AAS) and membrane-bound Na+ -K+ -ATPase activity (measured by UV-VIS with a Na+ -K+ -ATPase Detection Kit) of Eimeria tenella sporozoites of the sensitive line (i.e., the parent line, coded as OS) and 2 resistant lines, derived from the parent line (coded as OR125 and OR200), with and without in vitro exposure to monensin. These parameters for OR125 and OR200 were significantly lower than those for OS. In vitro exposure to monensin increased intrasporozoite Na+/K+ concentrations and Na+ -K+ -ATPase activity, but the stimulation on OS was significantly higher than those on OR125 and OR200, indicating that monensin had less effect on resistant parasites. The results of this study suggest that altered biochemical or physiological properties, or both, in the membranes of E. tenella might be related to a reduced sensitivity to monensin.     

19(S)-hydroxyeicosatetraenoic acid is a potent stimulator of renal Na+-K+-ATPase

Omega- and omega-1 hydroxylations are the major pathways by which arachidonic acid is metabolized in cortical and outer medullary microsomes of rat and rabbit kidneys. It is a cytochrome P450-dependent oxidation leading to the formation of 20-hydroxy- and 19-hydroxyeicosatetraenoic acids. In this study, we compared the effects of the synthetically prepared omega- and omega-1 metabolites of arachidonic acid on the activity of the renal Na+-K+-ATPase partially purified from rat renal cortical microsomes. 19(S)-hydroxyeicosatetraenoic acid caused a dose related stimulation of Na+-K+-ATPase activity with an EC50 of 3 x 10(-7) M. In contrast, neither 19(R)-hydroxyeicosatetraenoic acid, 20-hydroxyeicosatetraenoic acid nor arachidonic acid at 10(-6) M had any effect on Na+-K+-ATPase activity. In the same preparation, ouabain at 10(-3) M and 12(R)-hydroxyeicosatetraenoic acid at 10(-6) M inhibited the enzyme activity by 75% and 60%, respectively. We conclude that 19(S)-hydroxyeicosatetraenoic acid is a specific stimulator of renal Na+-K+-ATPase. Therefore, the formation of 19(S)-hydroxyeicosatetraenoic acid by renal cortical cytochrome P450 omega-1-hydroxylase may contribute to the regulation of renal function by regulating Na+-K+-ATPase which is essential for transtubular transport processes.     

Involvement of PI 3-kinase in IGF-I stimulation of jejunal Na+-K+-ATPase activity and nutrient absorption

Mechanisms responsible for increased jejunal transport rates observed in tissues treated with orally administered insulin-like growth factor-I (IGF-I) were studied in 5-day-old colostrum-deprived piglets. Human recombinant IGF-I (3.5 mg. kg(-1). day(-1)) or control vehicle was given orogastrically for 4 days. Disaccharidase activity, fructose uptake, and Na+-glucose cotransporter SGLT-1 protein abundance were similar between groups. Oral IGF-I produced greater rates of enterocyte Na+-K+-ATPase activity with no significant differences in Na+-K+-ATPase abundance. Cellular mechanisms responsible for transport changes were studied in Ussing chambers. In control tissues, the presence of IGF-I in mucosal solutions increased basal short-circuit current (I(sc)), potential difference, D-glucose-stimulated I(sc), and Na+-K+-ATPase activity; these changes were abolished by preincubation of tissues with wortmannin, a phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor. The results suggest that the effect of IGF-I on jejunal ion and nutrient transport involves activation of PI 3-kinase and stimulation of Na+-K+-ATPase activity in enterocytes.