Vascular smooth muscle cells (VSMC) proliferation of streptozotocin-diabetic animals induced by diadenosine polyphosphates.

UNLABELLED: Specific binding sites for diadenosine polyphosphates (Ap (4)A, Ap (5)A, Ap (6)A) exist in VSMC (cultured vascular smooth muscle cells). These compounds may regulate VSMC growth and proliferation which is a key event in atherogenesis. Since diabetes is a known risk factor for atherosclerosis, the proliferation of VSMC from normoglycemic (control) and hyperglycemic (diabetic) rats were compared and the possibly involved receptors for diadenosine polyphosphates inducing this effect were investigated. Diabetes was induced by streptozotocin (66 mg/kg i.p.) and VSMC were prepared from rat aorta (primary culture). ( (3)H)thymidine incorporation was a measure of cell proliferation. For all diadenosine polyphosphates tested a stimulatory effect was observed as a bell-shaped concentration-response curve and a maximum effect at 10 micro M (physiological concentration). Ap (6)A has the most prominent effect (247.8 +/- 33.2 % increase over basal). In VSMC of diabetic rats the effects were even more prominent (Ap (5)A: 430.1 +/- 62.7 %). ATP (a degradation product of Ap (6)A) is able to increase the maximum effect of 10 micro M Ap (6)A. UTP (P2Y (2) agonist) exhibits a weaker proliferation. 1 micro M suramin (P2 receptor antagonist) shifts the concentration response curve of ATP and of Ap (6)A to the right. In contrast, 10 micro M PPADS (P2 X receptor antagonist) has no effect. There is no difference between VSMC of normal and diabetic rats in this respect. ADP, AMP, and adenosine exhibit a dual proliferative effect. The effect of either of these 3 compounds is much higher in VSMC of diabetic rats than of controls. 2MeSATP (P2Y (1) agonist) and alpha,beta-Methylen-ATP (P2X agonist) were not effective in VSMC of both normoglycemic and diabetic rats. IN CONCLUSION: The proliferative effect of diadenosine polyphosphates and some degradation products is more pronounced in VSMC of diabetic than of normal rats. Ap (6)A acts maximally by itself and not by its degradation product ATP. Adenosine receptors or an unknown P2Y (ApxA) receptor may be involved in proliferative effects, but not P2X and P2Y (1) receptors irrespective of a diabetic situation.

Diadenosine tetraphosphate (Ap4A) induces a diabetogenic situation: its impact on blood glucose, plasma insulin, gluconeogenesis, glucose uptake and GLUT-4 transporters.

Diadenosine polyphosphates such as Ap4A are physiologically released compounds for which both receptors as well as a role as second messengers for influencing insulin release have been shown. So far little is known about their pathophysiological impact on diabetes with respect to blood glucose and plasma insulin, glucose production via gluconeogenesis, glucose uptake and GLUT-4 expression. Rats given an intravenous bolus of Ap4A (0.75 mg/kg) developed a rapid and dramatic increase in blood glucose. Plasma insulin was only transiently increased (for 4 min), but did not follow the normally stimulatory effect of the elevated blood glucose. A bolus of 25 microg Ap4A quickly increased glucose release from perfused rat liver. Glucose uptake was reduced in 3T3 adipocytes. Reduced amounts of translocated GLUT-4 were found in 3T3 cell membranes incubated with 10 microM Ap4A. Thus, Ap4A itself induces a diabetic situation which is likely to be mediated by an increase in gluconeogenesis and/or an insulin resistance caused by a decrease in GLUT-4 and an attenuation of glucose uptake.