Effect of insulin on the glucose utilization in isolated cardiac myocytes from adult rat

Pharmacokinetic-pharmacodynamic modelling of human insulin: validity of pharmacological availability as a substitute for extent of bioavailability

A method for assessing the extent of bioavailability (EBA) of human insulin from pharmacological data was determined. The time course governing increases in the plasma concentration of immuno-reactive insulin (IRI), as well as its pharmacological effects (glucodynamics), was determined after the intravascular administration of varying doses of human insulin. Pharmacokinetic (PK), pharmacodynamic (PD), and link models were constructed to elucidate the quantitative relationship between plasma IRI levels and pharmacological effects. After extravascular administration of the test formulation, only the time course governing the observed pharmacological effects was determined. The pharmacological data was translated into theoretical plasma concentration data, using the PK-PD model. Following this, the area under the theoretical plasma concentration-time curve (AUC) of the test formulation was calculated. The EBA was then estimated against a reference (intravascular) formulation, using a conventional equation. Since the pharmacological effects of insulin were observed to be highly dosing-rate-dependent, the PD model used in this study was modified to apply over a wide range of infusion rates. The results of the PK-PD analysis indicate that the doses administered can be accurately predicted from pharmacological data. To validate this method, the EBAs of controlled release formulations (the Osmotic Mini Pumps) of insulin, subcutaneously administered to the rat, were estimated. The EBA values obtained (92-96%) fell within a reasonable range. The area under the effect-time curves (AUE) obtained following subcutaneous applications of the Osmotic Mini Pump were calculated in a model-independent manner, in addition to pharmacological availabilities (PA), which were estimated against the reference (intravascular) formulations. The estimated PA values varied from 312% to 78%, in accordance with the intravascular input rates of the reference formulations. This indicates that PA should not be used as a substitute for EBA, unless data involving similar intravascular dosing rates to that of the reference formulations is available.

Heterogeneous responses to an intravenous glucose load

The aim of this study was to determine how the insulin sensitive enzymes pyruvate dehydrogenase (PDH) complex and glycogen synthase (GS) of different tissues respond to an endogenous pulse of insulin elicited by an intravenous infusion of glucose. An infusion of glucose (0.5 g/kg) into conscious, unrestrained animals via an indwelling cannula rapidly elevated plasma insulin concentration (to approx. 600 microU/ml after 10 min). The animals were sacrificed at selected time points after the commencement of infusion. Samples of heart, red quadriceps muscle, white adipose tissue (WAT) and brown adipose tissue (BAT) were excised and assayed for PDH complex and GS activities. The glucose infusion elicited a rapid (< 5-10 min) increase in both PDH complex and GS activities in heart, BAT and WAT. The maximum rise in the activity of PDH and GS above basal were (respectively) 2- and 8-fold for heart, 5.5- and 5-fold for BAT, and 3.5- and 4-fold for WAT. The return of PDH complex activity to basal values was also very rapid (occurring over the next 20 min). The glucose infusion also stimulated GS activity in red quadriceps muscle but was, however without effect on PDH complex activity in this tissue. We conclude that although insulin stimulates PDH and GS with the same time course and magnitude in many insulin sensitive tissues, the time course and magnitude of insulin stimulation of these enzymes can vary between tissues. These results may mean that the stimulation of PDH complex and GS by insulin occurs via different receptor-effector pathways.

Effects of verapamil on the cardiac alpha 1-adrenoceptor signalling system in diabetic rats

We evaluated the effects of chronic verapamil treatment on the cardiac alpha 1-adrenoceptor signalling system in streptozocin-induced diabetic rats. The decrease in maximum cell surface [3H]bunazosin binding (Bmax) in isolated cardiac myocytes from the diabetic group (-46%, P < 0.01) was completely reversed by a 4-week course of verapamil, while Bmax in the verapamil-treated control group was unchanged. Similarly, the reduction in ventricular inositol 1,4,5-trisphosphate (IP3) production after stimulation with 10 microM noradrenaline (NA) seen in diabetes (-30%, P < 0.01) was completely normalized by verapamil, while the response in the verapamil-treated control group was unaffected. These results indicate that verapamil can induce complete recovery of the impaired cardiac alpha 1-adrenoceptor signalling system in the diabetic heart without affecting glucose metabolism.

Plasma membrane-specific deficiency in cardiac beta-adrenergic receptor in streptozocin-diabetic rats

Cell surface [3H]CGP 12177 binding sites in 10-wk streptozocin-diabetic rats decreased by 41% (P less than 0.01) compared with that in the control rats. In contrast, there was no difference in the total cell receptor concentration between the control and the diabetic rats, which was measured by hydrophobic antagonist [125I]-iodocyanopindolol binding. Forty-eight-hour in vivo insulin treatment significantly (P less than 0.05) increased cell surface beta-adrenergic receptor concentration by 37% above that in diabetic rats without any change in total receptor concentration in the cells. However in vitro treatment of 8 nM insulin, 33 mM glucose, or 10 mM 3-hydroxybutyrate for 2 h showed no effect on [3H]CGP 12177 binding. In contrast, 10 microM isoproterenol-dependent decrease and the recovery of cell surface receptors after the removal of the agonist were significantly (P less than 0.01) impaired in diabetic rats compared with those of control rats. These results indicate that only cell surface beta-adrenergic receptors decrease in diabetic rats, which may be associated with abnormalities in the receptor distribution. The decrease in cell surface receptor number closely associates with the diabetic state and is reversed by the short-term insulin treatment.

Transport and post-transport abnormalities of glucose metabolism in cardiocytes isolated from streptozotocin-induced diabetic rats

The altered glucose utilization in cardiocytes isolated from streptozotocin (STZ)-induced diabetic rats and its reversibility to normal following 2-week insulin treatment were studied. 3-O-Methylglucose (3-O-MG) transport in cardiocytes isolated from normal rats was stimulated to 150% (P less than 0.01) above the basal level in the presence of 80 nM insulin. The basal 3-O-MG transport in diabetic rats was decreased to 41% of that in normal rats, and further an increase in 3-O-MG transport was not observed even in the presence of 80 nM insulin. Similar significant decreases in glucose uptake in the presence of both trace (0.8 microM) and 5.5 mM glucose were also found in diabetic rats. However, in normal rats, lactate release into the media was stimulated by only 16% (P less than 0.05), and glucose oxidation was not stimulated in the presence of insulin, there being no significant difference between normal and diabetic rats. On 2-week insulin treatment of diabetic rats, both the basal 3-O-MG transport and glucose uptake in the presence of trace and 5.5 mM glucose returned to normal levels, with significant improvement of the blunted acute insulin action. In addition, both lactate release and glucose oxidation were also significantly (P less than 0.05) increased by in vivo insulin treatment. The maximum insulin-stimulated 3-O-MG transport (r = -0.79, P less than 0.01) and glucose uptake in cells in the presence of 5.5 mM glucose (r = -0.74, P less than 0.01) both showed a negative correlation with the plasma glucose concentration, but not with the plasma free fatty acid level or the plasma triglyceride level.(ABSTRACT TRUNCATED AT 250 WORDS)