Effects of organic mercurials on mammalian pancreatic -cells. Insulin release, glucose transport, glucose oxidation, membrane permeability and ultrastructure

The effects of p-chloromercuribenzoic acid and chloromercuribenzene-p-sulphonic acid on pancreatic islets were studied in vitro. Obese-hyperglycaemic mice were used as the source of microdissected islets containing more than 90% beta-cells. p-Chloromercuribenzoic acid and chloromercuribenzene-p-sulphonic acid stimulated insulin release at concentrations of 0.01mm or above. This stimulation was significantly inhibited by the omission of Ca(2+) or the addition of adrenaline, diazoxide or 2,4-dinitrophenol. p-Chloromercuribenzoic acid or chloromercuribenzene-p-sulphonic acid did not interfere with the insulin-releasing ability of glucose. Micro-perifusion experiments revealed that the release of insulin in response to organic mercurial occurred almost instantaneously, was reversible, and was biphasic. The two mercurials inhibited glucose transport as well as glucose oxidation, and increased the mannitol and sucrose spaces of isolated islets. Compared with the effects on insulin release, those on glucose transport and membrane permeability were characterized by a longer latency and/or required higher concentrations of organic mercurial. Apart from a seemingly higher proportion of beta-cells exhibiting certain degenerative features, in islets exposed to 0.1mm-chloromercuribenzene-p-sulphonic acid for 60min, no significant differences with respect to beta-cell fine structure were noted between non-incubated islets and islets incubated with chloromercuribenzene-p-sulphonic acid or glucose or both. It is suggested that insulin release may be regulated by relatively superficial thiol groups in the beta-cell plasma membrane.

Specificity of leucine stimulation of insulin release

The ability of leucine to stimulate insulin release was studied in an in vitro system, using microdissected pancreatic islets with a high proportion of the insulin-producing <i>β</i>-cells. When tested in a glucose-free medium, L-leucine stimulated insulin release over a wide concentration range with maximum effect at about 20 mM. The stimulation of insulin release was additive to that elicited by 10 mM glucose. The L-configuration was essential for leucine to be recognized by the <i>β</i>-cells as an insulin secretagogue. L-isoleucine, alone or in combination with L-leucine, did not affect insulin release. It seems, therefore, unlikely that all sites for L-leucine transport are identical with receptor sites for stimulation of insulin release.

Visualization in freeze-dried tissue sections of structures to be studied by quantitative histochemistry

The limiting factor for quantitative histochemistry employing freeze-dried tissue sections is the ability to identify morphologic structures. Satisfactory staining of freeze-dried tissue specimens intended for further microanalysis was achieved by using isopentane solutions of free dye bases. The practical details of this rapid histologic control are outlined and its usefulness is illustrated by measurements of fructose 1,6-diphosphate and insulin in microdissected specimens of pancreatic β cells.

Effects of gastrin on the release of insulin in vitro

Several investigations in vivo and in vitro have shown that gastrointestinal hormones stimulate insulin secretion. Whether gastrin also has such an effect was tested both with the isolated mouse pancreas and with micro-dissected pancreatic islets from obese-hyperglycaemic mice. A fairly low concentration of human synthetic gastrin I (0·15 μg./ml.) was found to inhibit the stimulation of insulin release normally obtained with increasing glucose concentrations. However, when a higher concentration of gastrin was tested on the isolated pancreas in the presence of a low glucose concentration there was a stimulation of insulin secretion.