3.3. Blood Circulation
[16] In spite of the well established negative inotropic
effect (i.e., reduction of contractility) of hypercapnia on fish
myocardium in vitro (seeFarrell and Jones[1992] for
review), in vivo cardiac responses to hypercapnia vary
among fishes, and so do the blood pressure responses (see
Perry and Gilmour[2002] for a review). Obviously, in vivo
cardiovascular responses to hypercapnia vary with the
severity as well as the duration of hypercapnia imposed
on the fish, let alone interspecific variability, and probably
the experimental temperature. We have recently shown that
cardiac output (blood volume pumped per unit time) decreased rapidly when yellowtail was exposed to a lethal
level of hypercapnia (seawater equilibrated with a gas
mixture containing 5% CO2
(Figure 3)). Heart rate did not
change during the CO2
exposure, and it was a reduction of
the stroke volume (blood volume pumped per contraction)
that caused a reduction in cardiac output. Cardiac output
began to fall before blood pressure started to rise [Lee et al.,
2003]. The high solubility of CO2
will quickly lower
intracellular pH of the myocardium, reducing contractility
through an antagonism between hydrogen ions and intracellular calcium ions [Gesser and Poupa, 1983]. Therefore