Respiratory-related discharge pattern of sympathetic nerve activity in the spontaneously hypertensive rat

1. Synchronization of spontaneous sympathetic discharge during the respiratory cycle was studied in the cervical and renal nerves of vagotomized, normotensive Wistar-Kyoto rats (WKYs) and age-matched spontaneously hypertensive rats (SHRs). Phrenic nerve discharge was used as an index of central inspiratory activity. 2. In normotensive Wistar-Kyoto rats depression of sympathetic activity appeared at the onset of inspiration reaching a minimum at mid-inspiration. Peak maximal sympathetic discharge corresponded to postinspiratory phase; a second increase sometimes appeared in late expiration. Variations of respiratory frequency over wide range of experimental conditions by hypoxia, hyperoxia, hyper- or hypocapnia and transection of carotid sinus nerves did not affect this pattern. 3. In SHRs the respiratory-phase-related timing of sympathetic discharge was variable. In normoxia, the maximal sympathetic activity occurred in late inspiration, preceded by short depression at early inspiration and followed by postinspiratory depression. A second increase in sympathetic activity was observed in mid-expiration. 4. The pattern of respiratory phase modulated sympathetic activity in SHRs was altered by hypoxic stimulation of the peripheral chemoreceptors. The early inspiratory depression of sympathetic activity was substantially prolonged and the maximal sympathetic discharge was shifted from inspiration to early expiration. This effect was abolished after carotid sinus nerves had been cut. 5. Hypercapnic stimulation of central chemoreceptors in SHRs with carotid sinus nerves cut did not influence the timing of the sympathetic activity in relation to the respiratory phase, though the magnitude of rhythmical sympathetic discharges was increased. 6. We discuss the possibility that altered synchronization between central respiratory drive and sympathetic neuronal system may contribute to the neurogenic mechanisms of arterial hypertension in SHRs.