They also showed in an animal model that amylin accumulation in the heart leads to heart muscle destruction and failure. The study also found amylin build-up in overweight patients who are not obese, suggesting the potentially dangerous accumulations may start before a diabetes diagnosis.
Heart failure is the number-one killer in obese and diabetic populations. Controlling the circulation of amylin hormone in the blood might lessen or prevent disabilities and deaths from heart disease, the scientists said.
"Amylin appears to be a stealth killer," said Florin Despa of UC Davis and senior author of the study. "There is only one amylin protein for every 100 insulin proteins in the blood, so it has been under the radar until recently." In healthy people, amylin circulates in the blood together with insulin --the hormone that controls carbohydrate and fat metabolism – and principally regulates gastric fluxes and the sensation of satiety.
In studies of both normal and failing donated hearts of people undergoing heart-transplant surgery, the researchers found little or no amylin accumulation in lean people. But a quite different picture emerged from examinations of heart tissue of obese and type 2 diabetic patients. In failing hearts of these patients, they found extensive accumulation of amylin in strings of 10 to 20 proteins called oligomers. They detected a smaller but still abnormal build-up in nonfailing hearts from patients who were overweight but not obese.
Using genetically engineered rats that secrete human amylin in the same proportion as it is found in obese people, the researchers determined that amylin oligomers attach to membranes of myocytes – the heart-muscle cells that control heart beats. This made the membranes more porous to calcium, which changed myocyte contractility, altered the expression of vital proteins and, eventually, caused heart muscle cells to die.
"The significantly altered cardiac myocyte structure and function in the rats, along with the high levels of oligomers in the human heart tissue, strongly suggest that amylin is a major contributor to heart failure in obese and diabetic patients," Despa said.
Despa thinks that the link between cardiac amylin accumulation and heart disease has been overlooked because amylin circulates in relatively small amounts in blood, and because animals, including rats, that are often used in most studies of diabetic cardiac dysfunction do not normally express the form of amylin that aggregates and builds up in tissues.
MEDICA.de; Source: University of California Davis