High Glucose Levels Alter Angiotensin II-Induced Ca²⁺ Uptake via PKC and cAMP Pathways in Renal Proximal Tubular Cells

Although a dysfunction of the calcium metabolism occurs in diabetes mellitus, alterations of Ca²⁺ uptake induced by angiotensin II (ANG II) in renal proximal tubular cells (PTCs) grown in high-glucose medium are not fully elucidated. Thus, we examined whether high glucose concentrations can induce an alteration of the ANG II effect on the Ca²⁺ uptake and its action mechanism in primary cultured renal PTCs. PTCs were exposed to different glucose concentrations (5–100 mM) and time intervals (0–48 h). There was a sustained increase of Ca²⁺ uptake at glucose concentrations >15 mM. Thus, we selected 25 mM glucose and incubation for 48 h to maintain a hyperglycemic condition in vitro, unlike short-time regulatin. ANG II significantly inhibited the Ca²⁺ uptake in a dose-dependent manner in a 5-mM glucose medium. In addition, downregulation of ANG II receptors appeared in a glucose dose dependent manner. However, PTCs treated with 25 mM glucose for 48 h, not 12 h, did not exhibit the inhibitory effect of ANG II (10–⁷ M) on Ca²⁺ uptake, although the inhibitory effect of ANG II on Ca²⁺ uptake occurred in the presence of 25 mM mannitol or L-glucose. Staurosporine, bisindolylmaleimide I (protein kinase C, PKC, inhibitors), 12-o-tetradecanoylphorbol 13-acetate pretreatment, SQ 22536 (an adenylate cyclase inhibitor), and myristoylated protein kinase A inhibitor amide 14–22 (a protein kinase A inhibitor) blocked the 25-mM-glucose-induced alteration of ANG II effect on Ca²⁺ uptake. These results suggest that both PKC and cyclic adenosine monophosphate (cAMP) pathways are involved in the high-glucose-induced alteration of ANG II effect on Ca²⁺ uptake. Indeed, 25 mM glucose increased PKC activity and cAMP contents. In conclusion, a high glucose concentration altered ANG II induced inhibition of Ca²⁺ uptake via PKC and cAMP pathways in the PTCs.