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Cardiac control in the Pacific hagfish (Eptatretus stoutii)

University of British Columbia

The Pacific hagfish (Eptatretus stoutii), being an extant ancestral craniate, possesses the most ancestral craniate-type heart with valved chambers, a response to increased filling pressure with increased stroke volume (Frank-Starling mechanism), and myogenic contractions. Unlike all other known craniate hearts, this heart receives no direct neural stimulation. Despite this, heart rate can vary four-fold during a prolonged, 36-h anoxic challenge followed by a normoxic recovery period, with heart rate decreasing in anoxia, and increasing beyond routine rates during recovery, a remarkable feat for an aneural heart. This thesis is a study of how the hagfish can regulate heart rate without the assistance of neural stimulation. A major role of hyperpolarization-activated cyclic nucleotide-activated (HCN) channels in heartbeat initiation was indicated by pharmacological application of zatebradine to spontaneously contracting, isolated hearts, which stopped atrial contraction and vastly reduced ventricular contraction. Tetrodotoxin inhibition of voltage-gated Na⁺ channels induced an atrioventricular block suggesting these channels play a role in cardiac conduction. Partial cloning of HCN channel mRNA extracted from hagfish hearts revealed six HCN isoforms, two hagfish representatives of vertebrate HCN2 (HCN2a and HCN2b), three of HCN3 (HCN3a, HCN3b and HCN3c) and one HCN4. Two paralogs of HCN3b were discovered, however, HCN3a dominated the expression of HCN isoforms followed by HCN4. All HCN isoforms bar HCN3b were dominantly expressed in the atrium, likely to support greater atrial excitability ensuring synchronous contractions. Phylogenetic analysis suggested that HCN3 is the ancestral isoform supporting previous observations. Studies with β-adrenoreceptor agonists and antagonists in isolated, spontaneously beating hearts showed that the routine normoxic heart rate may involve maximal catecholamine stimulation of heart rate through cAMP stimulation of HCN channels via transmembrane adenylyl cyclase (tmAC). Loss of this tonic β-adrenorecptor cardiac stimulation during anoxia reduces heart rate, but restoring β-adrenoreceptor stimulation during normoxic recovery does not produce the previously observed increase above routine heart rate in vivo. Instead, bicarbonate-stimulated, soluble adenylyl cyclase (sAC) mediated cAMP production was found to produce this tachycardia in addition to the reinstated tmAC produced cAMP. This is the first time sAC has been implicated in heart rate control.

Text

2014-10-28

Attribution-NonCommercial-NoDerivs 2.5 Canada

http://hdl.handle.net/2429/50915

Zoology

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/2.5/ca/