The effects of prolonged bromocriptine administration on PRL secretion GH and glycaemic control in stable insulin-dependent diabetes mellitus

Bromocriptine: old drug, new formulation and new indication

Bromocriptine is an ergot alkaloid dopamine D(2) receptor agonist that has been used extensively in the past to treat hyperprolactinaemia, galactorrhoea and Parkinsonism. It is known that hypothalamic hypodopaminergic states and disturbed circadian rhythm are associated with the development of insulin resistance, obesity and diabetes in animals and humans. When administered in the early morning at the start of the light phase, a new quick release (QR) formulation of bromocriptine appears to act centrally to reset circadian rhythms of hypothalamic dopamine and serotonin and improve insulin resistance and other metabolic abnormalities. Phase II and III clinical studies show that QR-bromocriptine lowers glycated haemoglobin by 0.6-1.2% (7-13 mmol/mol) either as monotherapy or in combination with other antidiabetes medications. Apart from nausea, the drug is well tolerated. The doses used to treat diabetes (up to 4.8 mg daily) are much lower than those used to treat Parkinson's disease and have not been associated with retroperitoneal fibrosis or heart valve abnormalities. QR-bromocriptine (Cycloset™) has recently been approved in the USA for the treatment of type 2 diabetes mellitus (T2DM). Thus, a QR formulation of bromocriptine timed for peak delivery in the early morning may provide a novel neurally mediated approach to the control of hyperglycaemia in T2DM.

Drug-induced changes in prolactin secretion. Clinical implications

Prolactin secretion is affected by various diseases as well as by many drugs in humans and animals. While marked hyperprolactinaemia suggests the presence of a pituitary tumor, moderate changes may also occur in various endocrine or non-endocrine disorders. Drugs can interfere with prolactin regulation via complex mechanisms at the hypothalamus or at the pituitary site, but possible changes in prolactin metabolism are poorly understood as yet. This survey of the literature up to June 1986 covers the influence of various groups of drugs and agents on the plasma prolactin level under various conditions. It contains information that will facilitate evaluation of whether hyper- or hypoprolactinaemia may result from therapeutic intervention or must be related to an underlying disease. It is obvious that more subtle changes can be revealed by the use of dynamic tests either to stimulate or to suppress prolactin secretion.

In vivo insulin antagonism but evanescent in vitro tissue effect in rats with growth hormone-secreting tumors

Rats bearing mammosomatotropic tumors have raised insulin but lowered glucose concentrations. To determine if growth hormone (GH) secreted by these tumors causes insulin antagonism, pancreatic suppression tests utilizing infusions (per kg per min) of glucose (8 mg), insulin (200 ng) and somatostatin (1.4 micrograms) for 130 min were performed. Although the steady state plasma glucose and insulin levels (mean of 90, 100, 110, 120 and 130 min samples) were similar in 8 control and 13 tumor-bearing rats, the decrease from the already depressed basal glucose concentration (mmoles/l +/- SE) in the tumor animals was less than in the controls (0.90 +/- 0.30 vs. 2.56 +/- 0.040, p less than 0.005). Since the interpretation of these results was not entirely clear, glucose and insulin-glucose tolerance tests were performed. The glucose disappearance rates (%/min +/- SE) in the glucose tolerance test were lower in 17 tumor rats (2.00 +/- 0.13) compared to 17 control animals (2.51 +/- 0.22). This difference just missed statistical significance (t = 2.00, value of 2.04 necessary for p = 0.05). The decrease occurred in the presence of increased insulin (nmoles/l X 16 min) levels (4.29 +/- 0.38 vs. 2.58 +/- 0.29, p less than 0.005) suggesting insulin antagonism. The glucose disappearance rates (%/min +/- SE) in the insulin-glucose tolerance test were less in 12 tumor-bearing rats compared to 11 control animals (2.80 +/- 0.29 vs. 4.12 +/- 0.35, p less than 0.02). Thus, these GH-secreting tumors cause insulin antagonism in vivo. Freshly isolated hepatocytes from these tumor-bearing animals manifest decreased insulin binding and action (Diabetologia 25:60, 1983). In the present study, however, insulin binding and action (net glucose-C14 incorporation into glycogen) were normal after the hepatocytes were cultured for two days. This suggests that the changes induced by GH in vivo that lead to insulin antagonism are short-lived.