Long-term therapy of IDDM with an implantable insulin pump. The Implantable Insulin Pump Trial Study Group.

OBJECTIVE: To examine the long-term benefits and risks of treatment of IDDM with an implantable programmable insulin pump. RESEARCH DESIGN AND METHODS: Seventy-six patients with IDDM were studied at nine clinical centers. After 3-4 months of intensive subcutaneous therapy, the Infusaid Model 1000 pump was implanted, and insulin was delivered either intraperitoneally or intravenously for an average of 39.6 +/- 10 months (251 patient-years). Data was collected for glycemic control, lipid levels, weight gain, insulin requirements, adverse events, and quality of life. Sixty-three patients were also followed for 8.5 +/- 6.3 months (45 patient-years) after pump therapy was discontinued. RESULTS: Mean quarterly HbA1c fell with subcutaneous intensive therapy and remained stable on implantable pump therapy between 6.9 and 7.5%. Severe hypoglycemia was relatively rare, with only 4 episodes/100 patient-years of implantable pump therapy. This rate was significantly less than with subcutaneous intensive therapy before implantable pump initiation (33 episodes/100 patient-years) or after discontinuation of implantable pump therapy (36/100 patient-years) (P < 0.003). Weight did not increase significantly in the 1st year of therapy, but increased by 2.0 +/- 4.3 kg after 3 years of therapy. There were no significant differences in metabolic control or adverse events between intraperitoneal and intravenous insulin therapy except for minor differences in lipid levels and the more frequent development of catheter obstruction with intravenous delivery. Most pump slow-downs and catheter occlusions were corrected noninvasively. Quality of life, as measured by the Diabetes Control and Complications Trial instrument, showed high satisfaction and improved impact scores. CONCLUSIONS: Long-term implantable pump therapy maintained HbA1c in a range similar to intensive subcutaneous therapy, but with fewer episodes of severe hypoglycemia. Although pump and catheter occlusions remain a limitation, patient satisfaction with implantable pump therapy remains high.

Low-affinity intestinal L-aspartate transport with 2:1 coupling stoichiometry for Na+/Asp.

Epithelial cells isolated from chick small intestine were used to define the ionic and electrical characteristics of a low-affinity (Km = 4.1 mM) L-aspartate transport system. L-Glutamate and D-aspartate, but not D-glutamate, were found to inhibit L-aspartate influx, suggesting that this uptake system has a substrate specificity similar to that previously described for a high-affinity (Km = 16 microM) acidic amino acid transporter in the same cells. Low-affinity uptake is Na+ dependent with a Hill coefficient (n) of 1.4. Intracellular K+ moderately enhances but is not required for aspartate influx, and this response is modulated by changes in intracellular pH. The Na+-dependent uptake of aspartate is electroneutral, as evidenced by insensitivity to pronounced changes in delta psi induced by anion gradients or valinomycin in the presence of K+ gradients. Because the above characteristics can be consistent with several transport models, direct measurement of delta Na+-delta Asp coupling stoichiometry were performed. The coupling ratio was determined to be approximately 2.0. A model for intestinal Na+-dependent L-Asp transport is suggested in which each transport cycle involves inward transfer of 2Na+:1Asp+ and outward transfer of K+ or H+ in a net electroneutral set of events.

Stabilization of myc proto-oncogene proteins during Friend murine erythroleukemia cell differentiation.

A rapid and dramatic decrease in c-myc mRNA has been associated with the differentiation of a variety of cell types and may be a critical step in the maturation process. In this study, we have simultaneously measured steady-state c-myc protein and c-myc mRNA levels in differentiating Friend murine erythroleukemia (MEL) cells. Northern blot analysis of poly(A+) RNA indicated a greater than 85% decrease in c-myc transcripts following a brief exposure to the inducing agents, dimethyl sulfoxide or hypoxanthine. The short half-life of the c-myc protein (approximately 30 min) suggests that its level should fall in a similar fashion. Surprisingly, immunoblots of total cell proteins and immunoprecipitations of 35S-labeled protein revealed that c-myc protein levels remain approximately constant. This unexpected finding was accounted for in part by an increase in the c-myc protein half-life to 75-85 min. Immunoprecipitation of [35S]methionine-labeled proteins also demonstrated that the undifferentiated MEL cell synthesizes equal amounts of two c-myc-related proteins, p59 and p61. In contrast, MEL cell populations in the late stages of differentiation express predominantly the higher molecular weight species. This latter observation represents the first report of a temporal shift in the relative abundance of the two c-myc isoforms during cell differentiation.

Down-regulation of protein kinase C and of an endogenous 80-kDa substrate in transformed fibroblasts.

Subconfluent cultures of NIH-3T3 fibroblasts transformed by the Ha-ras, Ki-v-ras, v-src, and v-fms oncogene proteins all possess elevated steady-state levels of diacylglycerol, the endogenous activator of protein kinase C, as compared to the nontransformed parental lines. These oncogene-transformed fibroblasts also exhibit a significantly decreased level of cellular protein kinase C activity as measured by four different criteria: phorbol ester-stimulated phosphorylation of an endogenous 80-kilodalton (80 kDa) substrate; phorbol ester-stimulated changes in 86Rb uptake; enzymatic assay; and [3H]phorbol ester binding. In all cases, the transformed cells demonstrated an attenuated response to phorbol ester addition and a lower phorbol ester binding capacity as compared to the parental lines. Western analysis of the endogenous 80-kDa substrate of protein kinase C revealed a significantly lower level of this protein in the transformed cells than in the untransformed controls, and this decrease could be mimicked in parental cells by long-term incubation with phorbol esters, suggesting that the level of the 80-kDa protein is regulated by the state of activation of protein kinase C. These effects do not appear to be nonspecific responses to autocrine secretions by the transformed cells. They may represent an unsuccessful attempt by the transformed cells to negatively modulate the constitutive proliferative signals generated by the oncogene products.

High affinity L-aspartate transport in chick small intestine.

Epithelial cells isolated from chick small intestine were used to study the mechanism of L-aspartate transport. Two kinetically distinct uptake systems of high (Km' = 16 microM) and low (Km'' = 2.7 mM) affinity are observed. This paper examines the cation dependence and membrane potential sensitivity of the high affinity system. Unidirectional influx studies indicate that extracellular Na+ is an absolute requirement for transport function. Flux is optimal when K+ is present intracellularly, however this cation is not required for Na+-dependent L-aspartate uptake. In the absence of K+, flux enhancement is observed when the intracellular pH is acidic. In contrast, acidic intracellular pH is inhibitory in cells that are preequilibrated with K+. Sodium ([Na+]o greater than [Na+]i gradients, and potassium ([K+]o less than [K+]i) or proton ([H+]o less than [H+]i) gradients can independently energize the Na+-dependent accumulation of L-aspartate above equilibrium levels, suggesting that Na+ and L-aspartate cotransport occurs with concomitant K+ or H+ antiport. L-Aspartate influx is insensitive to membrane potential changes created by inwardly directed anion gradients in the presence or absence of intracellular K+. A model is presented that is consistent with electroneutral Na+-coupled transfer with an ion antiport site of low specificity.