Sydney S. Lazarus

Demonstration of glucose-6-phosphatase in mammalian pancreas.

Summary Glucose-6-phosphatase has been demonstrated in pancreatic B cells of rabbit. guinea pig and dog. and in canine ductular epithelium. Short fixation of fresh frozen cryostat sections in cold neutral 6% calcium formol does not inhibit its activity or alter its distribution in liver or pancreas. Furthermore, specificity of histochemical reaction for glucose-6-phosphatase was demonstrated by differences in staining when other substrates were incubated at same pH for equal time. In view of known relationship of blood sugar concentration to rate of insulin secretion, a hypothesis is advanced relating presence of this enzyme in B cells to control of insulin secretion. It is suggested that net rate of glucose-6-phosphate formation is the controlling factor and that this would be influenced by any increase or decrease of glucose-6-phosphatase activity.

Oxidative pathways in pancreatic B cells.

Summary 1) A histochemical study has been conducted of the distribution of glucose-6-phosphate dehydrogenase (G-6-PD), triphosphopyridine nucleotide diaphorase (TPND), lactic dehydrogenase (LD), diphosphopyridine nucleotide diaphorase (DPND) and succinic dehydrogenase (SD) in the pancreas of the rabbit. G-6-PD was found principally in B cells and duct epithelium; LD in duct epithelium, acinar tissue and A cells. TPND was present throughout, with greatest activity in B cells and in duct epithelium and good activity in other areas. SD activity was present throughout the pancreas with greatest staining in B cells and duct epithelium. The reactive structures for SD seemed to be mitochondrial while the other enzymes apparently are mostly extramitochondrial. 2) These findings demonstrate that glucose metabolism in B cells is predominantly via the hexose monophosphate shunt, and indicates a role for it in insulin synthesis. A relationship between TPNH generation by this pathway and availability of reduced glutathione or of free SH groups in enzymes is considered as a possible mechanism controlling B cell insulin output.

Acid and glucose-6-phosphatase activity of pancreatic B cells after cortisone and sulfonylureas.

Summary Glucose-6-phosphatase (G-6-Pase) and acid phosphatase (Ac-Pase) activities have been followed histochemically in hyperfunctioning rabbit pancreatic B cells in cortisone diabetes and after administration of hypoglycemic sulfonylureas. Cortisone diabetic rabbit pancreas with B cell degranulation and glycogenic vacuolization of ductules and B cells showed no definite changes in B cell Ac-Pase or G-6-Pase activity. Hypoglycemic sulfonylureas administered over period adequate to degranulate B cells, seemed to diminish their G-6-Pase content without effecting Ac-Pase. Although B cell degranulation was, in each of these instances, indicative of increased insulin output, lack of alteration of histochemically demonstrable B cell Ac-Pase activity was not considered to be at variance with the view that this enzyme may be important for insulin synthesis. Inhibition of B cell G-6-Pase activity by chronic sulfonylurea administration could be the mechanism of pancreatic action of these drugs. However, the alternative explanation that reduced G-6-Pase activity was a secondary adaptive response cannot be entirely excluded.

Comparison in Rabbits of Hypoglycemia and B Cell Degranulation After Various Sulfonylureas.

Summary 1) A single injection of same dose of 3 sulfonylureas, tolbutamide, chlorpropamide and metahexamide showed that tolbutamide caused a lesser hypoglycemia, whereas the 2 other drugs caused a more severe lowering of blood sugar level, most marked after administration of chlorpropamide. In no case was any B cell degranulation observed. 2) Infusion of glucose alone or simultaneously with various drugs for 7 hours each day on 3 successive days caused an identical hyperglycemic response in all animals. However, infusion of glucose alone produced no B cell degranulation; when sulfonylureas were added to the perfusate, degranulation of B cells was found. This was most pronounced after administration of chlorpropamide and metahexamide. That absence of hypoglycemia did not prevent B cell degranulation is further proof that degranulation of B cells after sulfonylureas is a primary action of these drugs on B cells to increase insulin output. Furthermore, relative B cell degranulating efficacy of the 3 compounds, in accord with their hypoglycemic effectiveness, supports the conclusion that they act directly on the B cell.