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Biomedical subjects

J R Cook

Publications and source records attributed to J R Cook.

At least 145 records · Page 8Linked to original sources
PubMed1974

Irreversible Plastid Loss in Euglena gracilis under Physiological Conditions.

Irreversible loss of the ability to develop chloroplasts in Euglena gracilis may develop following transfer from organic medium to defined medium. Requirements for the loss include the absence of light and a temperature of 30 C (the optimal temperature for multiplication) although neither darkness alone nor this temperature alone serves as the bleaching agent. The extent of bleaching of a population can approach 100% but depends heavily on the following conditions: the pH of the defined medium and its phosphate content, the age of the parent culture at transfer, and the length of time spent in the defined medium before cell divisions are permitted. Bleaching is not due to loss of nonreplicating proplastids through "dilution out" as cells divide but appears to be a more direct inactivation of chloroplast differentiation from proplastids.

Journal Article↗
PubMed1971

The role of adenosine 3':5'-cyclic monophosphate in the regulation of insulin release by isolated rat islets of Langerhans.

1. Concentrations of cyclic AMP (adenosine 3':5'-cyclic monophosphate) and rates of insulin release were measured in islets of Langerhans isolated from rat pancreas and incubated for various times in the presence of glucose, 3-isobutyl-1-methylxanthine, caffeine, theophylline, adrenaline and diazoxide. 2. Caffeine and theophylline produced small but significant increases in both cyclic AMP and release of insulin when they were incubated in the presence of 10mm-glucose. 3. 3-Isobutyl-1-methylxanthine produced a marked increase in the intracellular concentration of cyclic AMP in the presence of 5mm- and 10mm-glucose. However, insulin release was stimulated only in the presence of 10mm-glucose. 4. In response to rising concentrations of extracellular glucose (5-20mm) there was no detectable increase in the intracellular concentration of cyclic AMP even though there was a marked increase in the rate of insulin release. 5. In response to 10mm-glucose insulin release occurred in two phases and 3-isobutyl-1-methylxanthine potentiated the effect of glucose on both phases. The intracellular concentration of cyclic AMP remained constant with glucose and rose within 10min to its maximum value with 3-isobutyl-1-methylxanthine. 6. Adrenaline and diazoxide inhibited insulin release and lowered the intracellular concentration of cyclic AMP when islets were incubated with glucose or 3-isobutyl-1-methylxanthine. 7. It is suggested that glucose does not stimulate insulin release by increasing the concentration of cyclic AMP in islet cells. However, the concentration of cyclic AMP in islet cells may modulate the effect of glucose on the release process.

Animals↗