Cardiac dysfunction in diabetes is characterised in vitro by changes in contractility and reduced responsiveness to β-adrenergic stimulation. Alterations in the microtubular (MT) cytoskeleton have been implicated in cardiac dysfunction observed in pressure-overload hypertrophy and cardiac failure (Cooper, 2001) and there is evidence that, in certain circumstances, MTs may modify the response to β-stimulation (Calaghan et al. 2001). The purpose of this investigation was to determine whether changes in the cardiac MT cytoskeleton occur in a model of type 1 diabetes.

Diabetes was induced in male Wistar rats by I.P. injection of 60 mg kg^-1 streptozotocin (STZ). Rats were killed humanely (UK Home Office Schedule 1 methods) 2 months after injection, and ventricular myocytes were isolated enzymatically. Cells from each heart were stored in either 10 µM colchicine (a MT disruptor) or vehicle (0.1 % methanol) for 2 h, and then fixed and labelled with an antibody to β-tubulin and fluorescent secondary antibody (Howarth et al. 1999). Sections of 2 µm thickness through cells at the level of the nuclei were acquired by confocal laser microscopy. Quantitative analysis of fluorescent emission of images was used to provide an index of MT density. Diabetes was confirmed by measurement of blood glucose (STZ 235 ± 15 vs. control 60 ± 5 mg dl^-1; mean ± S.E.M.; n = 6). As shown in Fig. 1, fluorescence intensity was identical in vehicle-treated control (n = 72) and STZ cells (n = 70). Colchicine significantly decreased the number of MTs in control cells (n = 71) by 35 % (P < 0.001; Student’s t test). However, MTs in diabetic cells (n = 80) were unaffected (P > 0.05) by colchicine.We conclude that, whilst STZ-diabetes does not affect the number of MTs in cardiac cells, it generates MTs that are resistant to depolymerisation by colchicine. This implies a change to the normal cellular balance between a large population of dynamic MTs and a small subset of drug-stable MTs. The functional consequences of the change in structure or kinetics of MTs that renders them insensitive to colchicine could contribute to cardiac dysfunction in diabetes.This work was supported by the BHF and the British Council.

Figure 1. Fluorescence intensity of myocytes from control and STZ-diabetic rats labelled with antibody to β-tubulin. Open bars show vehicle-treated cells; shaded bars show colchicine-treated cells. ***P < 0.001 compared with vehicle-treated control cells.

Calaghan, S.C., Le Guennec, J.-Y. & White, E. (2001). Circ. Res. 88, e32-e37.

Cooper, G. IV (2001). Heart Failure Rev. 5, 187-201.

Howarth, F.C., Calaghan, S.C., Boyett, M.R. & White, E. (1999). J. Physiol. 516, 409-419. abstract