EFFECTS OF GENETIC OBESITY ON RAT UPPER AIRWAY MUSCLE AND DIAPHRAGM CONTRACTILE PROPERTIES

The contractile properties of pharyngeal respiratory muscle are altere d in sleep apnoea and in conditions associated with sleep apnoea, such as ageing. We hypothesized that the contractile properties of the pha ryngeal musculature are also altered by obesity, another factor associ ated with sleep apnoea. Studies compared a pharyngeal muscle, the ster nohyoid, with the diaphragm. These were chosen as representative muscl es whose contraction has opposing effects on upper airway patency. Bot h muscles were removed from nine lean and nine obese male Zucker rats (a genetic model of obesity), and isometric contractile properties wer e studied in vitro at 37 degrees C. For the sternohyoid muscle, in obe se compared to lean animals there were no significant differences in i sometric contraction time (15.2+/-0.3 vs 14.2+/-0.6 ms, respectively), half-relaxation time (13.6+/-0.5 vs 12.6+/-0.9 ms, respectively), twi tch-to-tetanic tension ratio (0.22+/-0.02 vs 0.24+/-0.02, respectively ), force-frequency relationship, fatigue resistance (2 min fatigue ind ex 0.20+/-0.03 vs 0.18+/-0.02, respectively), or maximal degree of for ce potentiation during repetitive stimulation (52+/-11 vs 74+/-20% inc rease, respectively). For the diaphragm, the only significant effect o f obesity was a lowering of the twitch-to-tetanic tension ratio (0.25/-0.01 vs 0.29+/-0.02, respectively). In obese, as in lean animals, th e sternohyoid had faster isometric twitch kinetics, a larger degree of force potentiation, and lower resistance to fatigue, than the diaphra gm. In lean, but not obese, animals the sternohyoid twitch-to-tetanic tension ratio was lower than and the force frequency relationship was located to the right of that of the diaphragm. In this study, genetic obesity in rats was not associated with any significant alterations in the contractile properties of the pharyngeal muscle, and only small c hanges in the relationship between the contractile properties of the s ternohyoid and diaphragm muscle.