A study of myocardial muscarinic receptors in streptozotocin-induced diabetic rats using iodine-123 N-methyl-4-iododexetimide

In previous studies we have shown that iodine-123 N-methyl-4-iododexetimide ([¹²³I]MIDEX) is a suitable single-photon emission tomography radiotracer for the characterisation of myocardial muscarinic acetylcholine receptors (m-AChR) in the normal state. It has been demonstrated that m-AChR are altered as a consequence of diabetes. The aim of the present study was to examine myocardial m-AChR density using [¹²³I]MIDEX in streptozotocin (STZ)-induced diabetic rats. In vitro binding experiments were conducted on left and right ventricle and atrium homogenate membranes of 1-week, 5-week and 10-week STZ-induced diabetic and aged-matched normal rats. The m-AChR densities (B _max values), as determined by saturation experiments with [¹²³I]MIDEX, revealed no difference in left and right ventricles or atrium in 1-week and 5-week STZ-diabetic rats when compared with normal rats. However, the 10-week STZ-diabetic group revealed a 39% (P<0.001) decrease in m-AChR density in atrium with no change in left and right ventricles. The equilibrium dissociation constant (K _d values) was similar in all groups. In vitro binding autoradiography revealed a 40% decrease in m-AChR density in atrium in the same 10-week diabetic rats. No statistically significant difference was found in 1-week and 5-week diabetic rats compared with normals. Ex vivo autoradiography showed a 50% decrease in [¹²³I]MIDEX uptake in atrium in 5-week diabetic rats and a 60% decrease in 10-week diabetic rats. These results demonstrate the ability of the single-photon agent [¹²³I]MIDEX to measure in vitro and ex vivo alterations in myocardial m-AChR density observed in STZ-induced diabetic rats. Received 16 November and in revised form 16 February 1999