Aims: 

Spontaneously hypertensive rats (SHR) are characterized by enhanced sympathetic tone, by reduced food intake (FI) and attenuated body weight (BW) development. These rats cannot reach the BW of normally fed controls even on a high-calorie diet. Increased sensitivity of the catabolic melanocortin (MC) system was also detected in SHR, what may contribute to their hypertension via enhancement of the sympathetic tone. These observations suggest alterations in the regulation of energy balance in SHR. To investigate such alterations, adaptation of SHR rats to fasting and their responsiveness to a centrally applied MC agonist were studied.

Methods: 

Parameters of energy balance [FI, BW, core temperature (Tc), heart rate (HR, indicating metabolic rate)] of 6-month-old male SHR were recorded in a biotelemetric system (MiniMitter) during a 7-day (1mg/ml/h) intracerebroventricular infusion of alpha-melanocyte-stimulating-hormone (alpha-MSH). In other rats adaptation to a 5-day fasting and following re-feeding was observed in the same system. Age- and BW-matched controls were used.

Results: 

Spontaneous FI of SHR was smaller than that of controls. During a 5-day fasting BW of all rats decreased similarly. However, SHR were unable to reach their original BW by the 5th day of re-feeding unlike controls, due to a reduction in their compensatory overeating. At the same time their adaptive suppression of Tc and HR were reversed sooner during re-feeding. Exogenous alpha-MSH-induced FI- and BW-reductions were similar in all rats, but the effects were of shorter duration in SHR. Elevations in HR and Tc were reduced and appeared during the active (nighttime) instead of the inactive (daytime) period.

Conclusions: 

Higher intrinsic MC activity contributing to the catabolic shift in the energy balance of SHR could not be further stimulated by exogenous alpha-MSH. However, this higher MC-activity may have contributed to the maladaptation to fasting-induced re-feeding. Support: OTKA PD84241, PTE AOK-KA-34039-02/2010