Role of SREBP-1 in the Development of Parasympathetic Dysfunction in the Hearts of Type 1 Diabetic Akita Mice
H. J Park,
C. M Welzig,
M. J Aronovitz,
S. P Georgescu,
Y. B Kim,
R. O Blaustein,
R. H Karas,
C. E Mathews
J. B. Galper
Rationale: Diabetic autonomic neuropathy (DAN), a major complication of diabetes mellitus, is characterized, in part, by impaired cardiac parasympathetic responsiveness. Parasympathetic stimulation of the heart involves activation of an acetylcholine-gated K+ current, IKAch, via a (GIRK1)2/(GIRK4)2 K+ channel. Sterol regulatory element binding protein-1 (SREBP-1) is a lipid-sensitive transcription factor.
Objective: We describe a unique SREBP-1–dependent mechanism for insulin regulation of cardiac parasympathetic response in a mouse model for DAN.
Methods and Results: Using implantable EKG transmitters, we demonstrated that compared with wild-type, Ins2Akita type I diabetic mice demonstrated a decrease in the negative chronotropic response to carbamylcholine characterized by a 2.4-fold decrease in the duration of bradycardia, a 52±8% decrease in atrial expression of GIRK1 (P<0.01), and a 31.3±2.1% decrease in SREBP-1 (P<0.05). Whole-cell patch-clamp studies of atrial myocytes from Akita mice exhibited a markedly decreased carbamylcholine stimulation of IKAch with a peak value of –181±31 pA/pF compared with –451±62 pA/pF (P<0.01) in cells from wild-type mice. Western blot analysis of extracts of Akita mice demonstrated that insulin treatment increased the expression of GIRK1, SREBP-1, and IKAch activity in atrial myocytes from these mice to levels in wild-type mice. Insulin treatment of cultured atrial myocytes stimulated GIRK1 expression 2.68±0.12-fold (P<0.01), which was reversed by overexpression of dominant negative SREBP-1. Finally, adenoviral expression of SREBP-1 in Akita atrial myocytes reversed the impaired IKAch to levels in cells from wild-type mice.
Conclusions: These results support a unique molecular mechanism for insulin regulation of GIRK1 expression and parasympathetic response via SREBP-1, which might play a role in the pathogenesis of DAN in response to insulin deficiency in the diabetic heart.