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High concentrations of protein tyrosine phosphatase (PTP) were found with secretory vesicles of glucagon-producing INR1G9 cells by electron microscopic immunocytochemistry, using a polyclonal antiserum specific for the PTP1B/T-cell (TC)PTP subfamily of PTP. Since TCPTP protein and mRNA were below the detection limit in the cells but significant amounts of PTP1B and mRNA were recognised by a specific monoclonal antibody and a mRNA probe we conclude, that the PTP associated with the vesicles is PTP1B. Only reverse transcriptase (RT)-PCR with primers specific for PTP1B yielded a product of the expected nucleotide sequence. Thus, we conclude that the PTP associated with the vesicles is PTP1B. The presence of vanadate for 48 h attenuated PTP1B expression and caused reduction of steady-state levels of the phosphatase. These conditions also led to a continuing increase in the steady-state rate of glucagon release by the cells. This rate and tyrosine phosphatase levels showed an inverse relationship, suggesting a suppressive role of PTP1B on the regulated secretion of glucagon by INR1G9 cells.

Elevated rates of steroid clearance may lead to lower reproductive success in several mammalian species. Cytochrome P450 (EC 1.14.14.1) and aldo-keto reductases (AKR; EC 1.1.1.145–151) are involved in the first phase of steroid inactivation, before second phase conjugation and excretion of the steroid metabolite. The current objectives were to determine liver blood flow (LBF), hepatic enzyme activity, and metabolic clearance rate (MCR) of progesterone (P₄) in dairy cows consuming isoenergetic and isonitrogenous diets formulated to cause divergent insulin secretion. Insulin concentrations increased by 22% in cows fed the high cornstarch diet, and both cytochrome P450 2C and cytochrome P450 3A activities were decreased (P<0.05) by ∼50%, while AKR1C tended (P<0.10) to be lower in cows fed the high cornstarch diet. LBF was similar between the two diets (1891±91 l/h). MCR of P₄ tended (P<0.10) to be lower in cows fed the high cornstarch diet (25±5 l/h×BW^0.75) versus the high fiber diet (40±6 l/h×BW^0.75). The half-life of P₄ was increased (P<0.05) in cows fed the high cornstarch diet (73±10 min) versus the high fiber diet (24±10 min). In summary, cows with elevated insulin concentrations and lower enzyme activity showed a decrease in P₄ clearance without any changes in LBF. This dietary relationship with hepatic enzyme activity may explain some of the observed alterations in steroid profiles during the estrous cycle or gestation of the high producing dairy cow.