Autonomic Control of Cardiac Function

Abstract

Cardiac parasympathetic tone mediates hypoxic bradycardia in fish, however the specific cholinergic mechanisms underlying this response have not been established. In Chapter 2, bradycardia in zebrafish (Danio rerio) larvae experiencing translational knockdown of the M2 muscarinic receptor was either prevented or limited at two different levels of hypoxia (PO2 = 30 or 40 Torr). Also, M2 receptor deficient fish exposed to exogenous procaterol (a presumed β2-adrenergic receptor agonist) had lower heart rates than similarly treated control fish, implying that the β2-adrenergic receptor may have a cardioinhibitory role in this species. Zebrafish have a single β1-adrenergic receptor (β1AR), but express two distinct β2-adrenergic receptor genes (β2aAR and β2bAR). Zebrafish β1AR deficient larvae described in Chapter 3 had lower resting heart rates than control larvae, which conforms to the stereotypical stimulatory nature of this receptor in the vertebrate heart. However, in larvae where loss of β2a/β2bAR and β1/β2bAR function was combined, heart rate was significantly increased. This confirmed my previous observation that the β2-adrenergic receptor has an inhibitory effect on heart rate in vivo. Fish release the catecholamines epinephrine and norepinephrine (the endogenous ligands of adrenergic receptors) into the circulation when exposed to hypoxia, if sufficiently severe. Zebrafish have two genes for tyrosine hydroxylase (TH1 and TH2), the rate limiting enzyme for catecholamine synthesis, which requires molecular oxygen as a cofactor. In Chapter 4, zebrafish larvae exposed to hypoxia for 4 days exhibited increased whole body epinephrine and norepinephrine content. TH2, but not TH1, mRNA expression decreased after 2 days of hypoxic exposure. The results of this thesis provide some of the first data on receptor-specific control of heart rate in fish under normal and hypoxic conditions. It also provides the first observations that catecholamine turnover and the mRNA expression of enzymes required for catecholamine synthesis in larvae are sensitive to hypoxia. Taken together, these data provide an interesting perspective on the balance of adrenergic and cholinergic control of heart rate in zebrafish larvae.