A prominent finding in the diabetic rat heart is a
decrease in the rate at which the ventricular muscle can
contract and relax. Since cardiac sarcoplasmic reticulum is
thought to be intimately involved in muscle contraction and
relaxation we studied the ability of diabetic rat cardiac
sarcoplasmic reticulum to transport Ca²⁺ . Hearts were obtained from female Wistar rats 7, 30, 42, and 120 days after the induction of diabetes by a single i.v. injection of either
alloxan (65 mg/kg) or streptozotocin (60 mg/kg). At all
Ca²⁺ concentrations tested (0.2 μM-5.0 μM free Ca²⁺)
cardiac sarcoplasmic reticulum obtained from 42 and 120 day
diabetic rats showed a significant decrease in the rate of
ATP-dependent tns-oxalate facilitated ²⁺ransport. This
was accompanied by a decrease in Ca²⁺ -ATPase activity. The
levels of long chain acylcarnitines associated with the
microsomal sarcoplasmic reticulum preparation from 42 and 120
day diabetic rats were significantly higher than those present
in sarcoplasmic reticulum from control rats.
Palmitylcarnitine, the most abundant of the long chain
acylcarnitines, in concentrations < 7 μM was found to be a
potent time-dependent inhibitor of both Ca²⁺ transport and
Ca²⁺ -ATPase in both control and diabetic rat sarcoplasmic
reticulum preparations; inhibition of Ca²⁺ transport was found to be more marked in the control preparations. This would indicate that a degree of inhibition produced by the high endogenous levels of palmitylcarnitine may already be
present in the diabetic rat heart preparations. Cardiac
sarcoplasmic reticulum prepared from acutely diabetic rats (7
days) did not show any decrease in Ca²⁺ transport ability.
Levels of long chain acylcarnitines associated with the
microsomal preparation enriched in sarcoplasmic reticulum were
also unchanged. Insulin treatment of diabetic rats could
significantly increase the ability of cardiac sarcoplasmic
reticulum to transport Ca²⁺, although at the time period
obtested
(30 days) the SR Ca²⁺ transport activity was only
slightly depressed as compared to control. Insulin treatment
also resulted in a slight, but non-significant, lowering of
the levels of long chain acylcarnitines associated with the
sarcoplasmic reticulum microsomal preparations. These
findings suggest that the alteration in sarcoplasmic reticulum
function in chronically diabetic rats may be due to the
buildup of cellular long chain acylcarnitines which inhibit
sarcoplasmic reticulum Ca²⁺ transport. The absence of any
significant change in Ca²⁺ transport activity or levels of long chain acylcarnitines at 7 and 30 days suggests that the alterations in 42 and 120 day diabetic rats must be of gradual onset.
Cardiac sarcoplasmic reticulum is known to be regulated
by a number of factors, among them calmodulin, cAMP-dependent
protein kinase, and K⁺. Since Ca²⁺ transport activity in cardiac sarcoplasmic reticulum from chronically diabetic rats is depressed, the role that these regulators play was investigated. Calmodulin (0.61 μM), cAMP (10 μM) plus cAMP-dependent protein kinase (0.2 mg/0.5 ml), and K⁺ (0-110
mM) all stimulated Ca transport in both control and
streptozotocin-treated diabetic rats to the same degree. This
suggests that the depression observed in sarcoplasmic
reticulum function from diabetic rats is not due to altered
regulation by these putative mediators of Ca²⁺ uptake.
A number of studies suggest that carnitine administration
may lower myocardial levels of long chain acylcarnitines in
the diabetic rat. Therefore, D,L-carnitine (1 g/kg/day,
orally) was administered to 120 day diabetic rats for a 30 day
period. The elevated levels of long chain acylcarnitines
normally seen in diabetic rats were significantly reduced in
the diabetic rats administered carnitine. Carnitine
administration, however, could not reverse the previously
noted depression in diabetic rat heart function, as measured
on an isolated working heart apparatus. In an effort to
prevent the onset of the diabetic cardiomyopathy D,L-carnitine
was administered (3 g/kg/day, orally) 3 days after the
induction of diabetes for a 42 day period. As previously
mentioned, sarcoplasmic reticulum Ca²⁺ transport activity
was depressed in diabetic rats, as compared to control rats,
at all free Ca²⁺ concentrations tested (0.1 μM-3.5 μM).
Similarly, sarcoplasmic reticulum levels of long chain
acylcarnitines were significantly elevated in these diabetic
rats. The diabetic rats treated with carnitine did not show
any depression in Ca²⁺ transport activity; long chain acylcarnitine levels were also similar to control. The carnitine-treated diabetic rats, however, showed no improvement in heart function compared to untreated-diabetic
rats. These data suggest that although the long chain acylcarnitines are inhibiting cardiac sarcoplasmic reticulum function in chronically diabetic rats other factors must also be contributing to the depression in heart function. / Pharmaceutical Sciences, Faculty of / Graduate
| Identifer | oai:union.ndltd.org:UBC/oai:circle.library.ubc.ca:2429/24314 |
| Date | January 1983 |
| Creators | Lopaschuk, Gary David |
| Publisher | University of British Columbia |
| Source Sets | University of British Columbia |
| Language | English |
| Detected Language | English |
| Type | Text, Thesis/Dissertation |
| Rights | For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use. |
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