The SA Journal Diabetes & Vascular Disease Vol 14 No 2 (December 2017)

RESEARCH ARTICLE

SA JOURNAL OF DIABETES & VASCULAR DISEASE

VOLUME 14 NUMBER 2 • DECEMBER 2017

D rats observed between 15 and 25 minutes of the test, we noted

that melatonin treatment had no significant effect on basal glucose

levels and the overall area under curve in both groups (Fig. 5).

Discussion

Our aim was to investigate the effect of melatonin treatment on

basal glucose uptake and insulin responsiveness as indicated by

glucose uptake, using cardiomyocytes isolated from young control

rats, age-matched controls and obese, insulin-resistant rats. The

results indicated that (1) melatonin treatment

in vitro

had no effect

on glucose uptake but increased insulin-stimulated glucose uptake

by cardiomyocytes from only the young and age-matched control

rats (Fig. 1B, Table 1); (2) melatonin treatment

in vivo

increased

basal and insulin-stimulated glucose uptake by cardiomyocytes

isolated from the hearts of obese, insulin-resistant rats.

During the basal state, glucose transport is commonly considered

the rate-limiting step for muscle glucose metabolism.

The

involvement of melatonin in glucose uptake was supported by the

observation that pinealectomised animals develop insulin resistance

associated with a decrease in glucose uptake by adipose tissue.

15,38

Accordingly, administration of melatonin reversed pinealectomy-

induced insulin resistance and improved glucose uptake by isolated

adipose tissue.

15,38

In contrast to this, our data show that melatonin

per se had no significant effect on

in vitro

glucose uptake by

cardiomyocytes isolated from young normal or obese rats and their

age-matched controls (Fig. 1A, Tables 1, 2). A similar observation

was previously reported in rat skeletal muscle cells

and chick

brain,

as well as in adipose tissue from a female fruit bat.

Of interest was our finding that acute melatonin administration

vitro

enhanced insulin-stimulated glucose uptake by cardiomyocytes

from normal young rats (Fig 1B) as well as the control rats fed for

16 to 19 weeks (Fig. 2). The enhanced insulin responsiveness of

glucose uptake may be related to a synergistic interaction between

melatonin and insulin action, supporting the insulin-sensitising

effect by melatonin, as previously demonstrated.

39,41,42

The

in vitro

melatonin-enhancing effect on insulin-stimulated

glucose uptake was not observed in cardiomyocytes isolated from

either the control or obese groups fed for more than 20 weeks

(Fig. 3), indicating a progressive loss of the synergistic interaction

between melatonin and insulin action. Although this is difficult

to explain, it may have resulted from ageing in the control group,

as previously demonstrated.

On the other hand, cardiomyocytes

from obese animals fed for 16 to 19 weeks were almost as insulin-

responsive as the control cardiac myocytes, but did not exhibit the

potentiating effect of melatonin compared to the control group.

Various physiological factors such as an effect on adiponectin and

leptin may have contributed to the overall effect of

in vivo

melatonin

on glucose uptake, as previously discussed.

In a preventative-

treatment setting, 16 weeks of melatonin consumption, starting

before the establishment of obesity, reduced hypertriglyceridaemia

and increased high-density lipoprotein cholesterol levels in rats

fed the same high-calorie diet.

However, the exact mechanism

whereby

in vivo

melatonin treatment affects glucose homeostasis

and enhances insulin responsiveness is complex and not fully

elucidated.

Melatonin induced a significant reduction in body weight,

associated with a concomitant increase in basal glucose uptake

by isolated cardiomyocytes from the obese rats. This effect is

consistent with previous observations that chronic melatonin

treatment reduced body weight gain and insulin resistance in

mice

and rats

fed a high-fat diet, as well as in old obese

and

young Zucker diabetic fatty

rats. Therefore, melatonin action may

involve melatonin receptors and various indirect effects on the

liver, pancreas and other peripheral insulin-sensitive organs, such

as adipose tissue and skeletal muscle.

A recent report shows that

the removal of melatonin receptors (MT1 or MT2) in mice abolished

the daily rhythm in blood glucose levels,

confirming the role of

melatonin signalling in the control of glucose homeostasis.

Contrary to the

in vitro

situation, melatonin administered

in vivo

increased basal glucose uptake by cardiomyocytes isolated from

obese rats. Mechanistically, this may involve glucose transporter

1 (GLUT1), which is usually associated with basal glucose uptake

Figure 4.

Effect of

in vivo

melatonin treatment (for the last six weeks of feeding)

on insulin-stimulated glucose uptake by cardiomyocytes isolated from rats fed a

high-calorie diet (20 weeks). Cardiomyocytes were isolated and stimulated with

increasing concentrations of insulin for a period of 30 minutes. The accumulated

radiolabelled 2DG was measured and expressed as pmol/mg protein/ 30 min. Ins:

insulin, C: control, CM: control with melatonin, D: high-calorie diet (dietinduced

obesity), DM: diet with melatonin. Treated vs untreated (same dose of insulin or

basal): *

< 0.05 (DM vs D), **

< 0.01 (CM vs C). Different doses of insulin vs

basal (same group of treatment):

< 0.05 vs basal,

< 0.01 vs basal,

###

0.001 vs basal. C vs D (same dose of insulin): and

< 0.05 (D vs C). Comparison

between different doses of insulin (same group of treatment):

< 0.01 vs 1 nM

Ins,

= four to six individual preparations/group; analysed in duplicate.

Figure 5.

Effect of

in vivo

melatonin treatment (for the last six weeks of feeding)

on intraperitoneal glucose tolerance. C: control, CM6: control with six weeks’

melatonin treatment, D: high-calorie diet (diet-induced obesity), DM6: high-

calorie diet with six weeks’ melatonin treatment, *

< 0.05 (D vs C),

< 0.05 (D

vs DM6),

= six per group.