Institutionen för fysiologi och farmakologi / Department of Physiology and Pharmacology
Abstract
Obesity and type 2 diabetes are major and rapidly increasing health
problems in society. They are associated with several life-threatening
conditions, including heart and renal failure, and damage to the nervous
system. An inability of cells to respond normally to insulin, insulin
resistance, is a key feature in obesity and type 2 diabetes.
Ca2+ is a versatile messenger that regulates diverse cellular functions
such as fertilization, electrical signaling, contraction, synaptic
transmission, gene transcription, hormonal signaling, metabolism, and
cell death. To exert these diverse effects, duration, amplitude and
spatial distribution of Ca2+ need to be tightly regulated. The role of
Ca2+ in insulin signaling under normal conditions and in association with
insulin resistance is uncertain.
This thesis focuses on Ca2+ fluxes and insulin action in cardiac and
skeletal muscles. In the first two papers we examined the effect of
insulin on Ca2+ homeostasis in normal, freshly isolated mouse ventricular
cardiomyocytes and how Ca2+ handling was changed in an animal model of
obesity and insulin resistance, ob/ob mice. Ob/ob cardiomyocytes showed
prolonged electrically evoked Ca2+ transients and impaired mitochondrial
Ca2+ handling, which resulted in extra Ca2+ transients that may
predispose for arrhythmias in vivo. Moreover, we observed decreased ion
fluxes through canonical transient receptor potential 3 (TRPC3) channels,
which may affect intracellular Ca2+ homeostasis and hence cellular
function.
In the following two papers, we investigated the role of Ca2+ in
insulin-mediated glucose uptake in adult skeletal muscles. Increased Ca2+
influx in the presence of insulin potentiated glucose uptake in muscles
from both normal and ob/ob mice, whereas decreased Ca2+ influx was
associated with decreased insulinmediated glucose uptake. In addition,
TRPC3 protein expression was knocked down using a novel transfection
technique with small interfering RNA coupled to carbon nanotubes, which
resulted in large decreases in diacylglycerol-induced Ca2+ influx and
insulin-mediated glucose uptake. Insulin-mediated glucose uptake occurs
via the glucose transporter 4 (GLUT4) that was found to co-localize with
TRPC3 in the t-tubular system, which is considered to be the predominant
site of glucose uptake in skeletal muscle.
Taken together, these studies shed light on how insulin and Ca2+ interact
in signaling in cardiac and skeletal muscles. In the heart, components
and channels that alter intracellular Ca2+ handling and might be involved
in the development of acute cardiac failure in insulin resistant
conditions have been identified. Further, we demonstrate that Ca2+ is
important for insulin-mediated glucose uptake. Thus, the present data
identify specific sites for therapeutic intervention in the treatment of
conditions associated with insulin resistance