As is well known, diabetes-induced cardiovascular complications are leading factors of morbidity and mortality. Among them atherosclerosis, hypertension and heart failure are most important ones. The very first problem seen in diabetic heart is the decrease of beta adrenoceptor mediated cardiac responses. The decrease of catecholamine stimulated beta adrenoceptor responsiveness is mostly associated with diminished receptor sensitivity and density. Insulin treatment of diabetic rats has been demonstrated to correct these parameters. However, the improving effect of insulin is shown to be abolished in tyhroidectomized diabetic rats. Although the effect of the relationship between these two hormones on some pathways has already been studied, their exact role on beta adrenoceptor mediated cardiac responses is uncertain. Thus in the present study, we aimed to determine the possible relationship between insulin and thyroid hormones to normalization of diabetes induced cardiac abnormalities.

Male Sprague-Dawley rats were used for this study. Ten days after surgical thyroidectomy, diabetes was induced with 38mg/kg streptozotocin by a single intravenous tail-vein injection. After 6-weeks of-diabetes induction, some groups (D-I, TxD-I) were treated with insulin (5–20U/kg/day, subcutan), some other groups (TxD-I-T2.5, TxD-I-T5) were administered insulin + thyroid hormone combination (2,5 or 5ug/kg/day T3 via osmotic minipump) for 2 weeks. Cardiac function was determined by the assessment of in vivo pressure-volume (PV) analysis and in vitro left ventricular papillary muscle experiments (isoprenaline, 0,0001uM-3uM; forskoline, 3uM). mRNA expressions of β1-AR and SERCA were evaluated by QPCR.

Heart rate and end systolic pressure were markedly reduced in diabetic (D) and thyroidectomized diabetic (TxD) groups; however, they were improved with insulin treatment in D group. On the other hand, in TxD group, only insulin and 5ug/kg T3 combination (TxD-I-T5) ameliorated these parameters. End diastolic pressure and ejection fraction were found to be unchanged between the groups. Rate of contraction and relaxation, time constant of left ventricle pressure decay were reduced in D and TxD groups. These parameters were corrected significantly in D-I and TxD-I-T5 groups. Cardiac index was reduced in D and TxD groups. This parameter was not increased markedly in any of the treated groups. End systolic volume index was raised in D and TxD groups, and reduced only in D-I group. End diastolic pressure index was also increased in D and TxD groups. It was significantly decreased in D-I and TxD-I-T5 group. Isoprenaline, a nonselective β-AR agonist, induced concentrationdependent positive inotropic effects on papillary muscles of control rats (C). Maximum response (Emax) was markedly diminished in D and TxD groups. These parameters were ameliorated in D-I group significantly. Furthermore, in TxD-I-T5 group, the contractile response was enhanced compared to TxD group. Forskolin induced positive inotropic effect in all groups. The response was decreased in D and TxD groups. Insulin treatment improved this decrease in D-I and TxD-I-T5 groups. β1-AR mRNA levels were decreased both in D and TxD groups. Insulin treatment increased β1-AR mRNA levels. In TxD-I group insulin did not increase β1-AR mRNA levels. On the other hand, in TxD-I-T5 group β1-AR mRNA levels were significantly increased compared to TxD, TXD-I and TxD-I-T2.5 groups. SERCA mRNA levels were also found to be reduced in D and TxD groups. Insulin treatment did not correct the decrease in mRNA levels of SERCA of D and TxD groups. In TxD-I-T5 group, SERCA mRNA levels were significantly increased compared to TxD, TXD-I and TxD-I-T2.5 groups.

Both in vivo and in vitro cardiac experiments showed that heart function was impaired in D and TxD groups. Insulin exerted beneficial effects only in D-I and TxD-I-T5 groups. As no improvement was seen in TxD-I group, it could be thought that insulin could not reverse beta adrenoceptor mediated cardiac responses in thyroid deficiency. The slight but still insignificant recovery that we observed in TxD-I-T2.5 group could be interpreted with inadequacy of T3 dose.