Sleep-dependent changes in the control of heart rhythm in spontaneous hypertensive rats

Sleep entails changes in autonomic control of physiological functions. At the cardiac level, the baroreflex modulates autonomic activity forcing heart period (HP) to correlate positively with previous values of mean arterial pressure (MAP). We assessed whether this pattern of HP vs. MAP coupling varies between sleep states and in cardiovascular disease. Six male Spontaneously Hypertensive Rats (SHR) and six Wistar-Kyoto (WKY) controls were implanted with electrodes for sleep-state identification and an arterial catheter for pressure measurement. Animals were studied after one week during spontaneous episodes of rapid-eye-movement sleep (REMS) and non-rapid-eye-movement sleep (NREMS). Cross-correlation functions were computed on beat-to-beat HP and MAP sequences extracted from 30 s blood pressure recordings during each sleep state and low-pass-filtered to remove ventilatory fluctuations. Data reported are grand means ± SEM. MAP was higher in SHR than in WKY (p<0.001, ANOVA), while HP did not vary significantly between strains. During NREMS, HP correlated positively with previous MAP values without significant differences between strains. The highest correlation coefficient (0.28±0.05, SHR; 0.29±0.03, WKY) occurred with HP following MAP by 0.67±0.08 s (SHR) and 0.71±0.27 s (WKY). During REMS, HP did not correlate positively with previous MAP values but rather correlated negatively with subsequent MAP values, with significant differences between strains. The lowest negative correlation coefficient (-0.29±0.01, SHR; -0.37±0.01, WKY; p<0.005, t-test) occurred with HP preceding MAP (by 0.94±0.05 s, SHR; 0.75±0.06 s, WKY; p<0.05, t-test). In NREMS, the coupling between HP and MAP was consistent with a prevailing baroreflex control, whose extent did not vary in the SHR model of essential hypertension. The baroreflex was not the main modulator of cardiac autonomic activity during REMS. In this state, non-baroreflex central autonomic commands prevailed on the heart and were blunted in essential hypertension. REMS may thus unmask pathological derangements in cardiovascular control