introducing a novel mechanism to control heart rate in the ancestral pacific hagfish although neural modulation of heart rate is well established among chordate animals the pacific hagfish eptatretus stoutii lacks any cardiac innervation yet it can increase its heart rate from the steady depressed heart rate seen in prolonged anoxia to almost double its normal normoxic heart rate an almost fourfold overall change during the 1h recovery from anoxia the present study sought mechanistic explanations for these regulatory changes in heart rate we provide evidence for a bicarbonateactivated soluble adenylyl cyclase sacdependent mechanism to control heart rate a mechanism never previously implicated in chordate cardiac control