Cardiovascular Journal of Africa: Vol 28 No 6 (November/December 2017)

CARDIOVASCULAR JOURNAL OF AFRICA • Volume 28, No 6, November/December 2017

366

AFRICA

(100 nM), compared to the untreated group (DM: 50.1

1.7 vs

D: 32.1

5.1 pmol/mg protein/30 min,

0.01) (Fig. 4).

Effect of melatonin treatment

in vivo

on IPGT test

in insulin-resistant rats

A high-calorie diet increased basal fasting blood glucose levels

compared to the control diet (5.2

0.28 vs 6.4

0.17 mM,

0.05). Similarly, at the end of the test, group D rats continued

to have elevated glucose levels (4.5

0.2 vs 5.2

0.1 mM,

0.05), compared to the control group (Fig. 5). The area under the

curve was also elevated in group D rats, compared to the controls

(870.7

25.6 vs 761.8

27.7,

0.05) (Table 3). However,

despite a significant decrease in blood glucose levels in the

melatonin-treated 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

in 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.

Basal

Ins (1 nM)

Ins (10 nM) Ins (100 nM)

2DG (pmol/mg protein/30 mins)

###

Fig. 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 (diet-

induced 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.

120

Time (min)

2DG (pmol/mg protein/30 mins)

CM6

DM6

Fig. 5.

Effect of

in vivo

melatonin treatment (for the last six

weeks of feeding) on intraperitoneal glucose toler-

ance. C: control, CM6: control with six weeks’ mela-

tonin treatment, D: high-calorie diet (diet-induced

obesity), DM6: high-calorie diet with six weeks’ mela-

tonin treatment, *

0.05 (D vs C),

0.05 (D vs

DM6),

six per group.