Acute effect of calcium and insulin on hyperfiltration of early diabetes

Exaggerated impact of ATP-sensitive K(+) channels on afferent arteriolar diameter in diabetes mellitus

Experiments were performed to determine the involvement of ATP-sensitive K(+) channels (K(ATP) channels) in the renal afferent arteriolar dilation that occurs during the hyperfiltration stage of insulin-dependent diabetes mellitus (IDDM). IDDM was induced in rats by streptozotocin (STZ) injection, and adequate insulin was provided to maintain moderate hyperglycemia. Sham rats received vehicle treatments. Two weeks later, afferent arteriolar function was assessed using the in vitro blood-perfused juxtamedullary nephron technique. Baseline afferent arteriolar lumen diameter was greater in STZ rats (25.9 +/- 1.1 microm) than in sham rats (20.8 +/- 1.0 microm). Glibenclamide (3 to 300 microM) had virtually no effect on afferent arterioles from sham rats; however, this K(ATP) antagonist caused concentration-dependent afferent arteriolar constriction in kidneys from STZ-treated rats, restoring lumen diameter to 20.6 +/- 1.7 microm (P > 0.05 versus sham baseline). In both groups of rats, pinacidil (a cyanoguanidine K(ATP) agonist; 0.3 to 300 microM) evoked concentration-dependent afferent arteriolar dilation, indicating the functional expression of K(ATP) channels; however, lumen diameter was increased by 73% in STZ kidneys but only by 48% in sham kidneys. The gliben-clamide-sensitive afferent arteriolar dilator response to 1 microM PCO-400 (a benzopyran K(ATP) agonist) was also accentuated in STZ kidneys. These observations suggest that increases in both the functional availability and basal activation of K(ATP) channels promote afferent arteriolar vasodilation during the early stage of IDDM, changes that likely contribute to the etiology of diabetic hyperfiltration.

Functional impairment of renal afferent arteriolar voltage-gated calcium channels in rats with diabetes mellitus

Experiments were performed to test the hypothesis that diabetes mellitus is associated with impaired afferent arteriolar responsiveness to opening of voltage-gated calcium channels. Diabetes was induced by injection of streptozocin (65 mg/kg, i.v.) and insulin was administered via an osmotic minipump to achieve moderate hyperglycemia. Sham rats received vehicle treatments. 2 wk later, the in vitro blood-perfused juxtamedullary nephron technique was used to allow videomicroscopic measurement of afferent arteriolar contractile responses to increasing bath concentrations of either Bay K 8644 or K+. Baseline afferent arteriolar diameter in kidneys from diabetic rats (26.4+/-1.2 microm) exceeded that of Sham rats (19.7+/-1.0 microm). Bay K 8644 evoked concentration-dependent reductions in afferent diameter in both groups of kidneys; however, arterioles from Sham rats responded to 1 nM Bay K 8644 while 100 nM Bay K 8644 was required to contract arterioles from diabetic rats. The EC50 for K+-induced reductions in afferent arteriolar diameter was greater in diabetic kidneys (40+/-4 mM) than in kidneys from Sham rats (28+/-4 mM; P < 0.05). In afferent arterioles isolated by microdissection from Sham rats and loaded with fura 2, increasing bath [K+] from 5 to 40 mM evoked a 98+/-12 nM increase in intracellular Ca2+ concentration ([Ca2+]i). [Ca2+]i responses to 40 mM K+ were suppressed in afferent arterioles from diabetic rats (delta = 63+/-5 nM), but were normalized by decreasing bath glucose concentration from 20 to 5 mM. These observations indicate that the early stage of insulin-dependent diabetes mellitus is associated with a functional defect in afferent arteriolar L-type calcium channels, an effect which may contribute to suppressed afferent arteriolar vasoconstrictor responsiveness and promote glomerular hyperfiltration.