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Delayed depolarization of the cog-wheel valve and pulmonary-to-systemic shunting in alligators

Douglas A. Syme^1,*, Kurt Gamperl^2, and David R. Jones^2,

¹ Department of Biological Sciences, 2500 University Drive NW, University of Calgary, Calgary, Alberta, Canada T2N 1N4
² Department of Zoology, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4
Present address: Ocean Sciences Centre, Memorial University of Newfoundland, St John's, Newfoundland, Canada A1C 5S7
Present address: Distinguished Scholar, Peter Walls Institute for Advanced Studies, The University Centre, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4

View larger version (22K): [in a new window]   Fig. 1. The crocodilian central circulation, ventral view. The right ventricle (RV) maintains connections to both the pulmonary circuit via the pulmonary artery (PA) and the systemic circuit via the left aorta (LAo), which continues as the coeliac artery (CA). The subpulmonary conus contains a muscular cog-wheel valve (CWV, made of cartilagenous teeth and surrounded by cardiac muscle), the contraction of which can occlude the entrance to the pulmonary artery. The right aorta (RAo) receives blood from the left ventricle (LV), gives rise to the common carotid artery (CCA) and the right/left subclavian arteries (R/LSA) and then continues as the dorsal aorta (DAo). The left and right aortas connect twice, just outside the ventricles through the foramen of Panizza (FP) and behind the heart via an anastomosis (JJ).

View larger version (23K): [in a new window]   Fig. 2. Electrocardiograms (ECGs) and pressures in the alligator pulmonary circuit. Top traces, ECGs recorded from the middle of the right ventricle (RV ECG, red) and the cog-wheel valve muscle (cog-wheel ECG, blue). Bottom traces, right ventricular pressure (RV, red) and pulmonary arterial pressure (PA, green). A and C show results from animals with a functioning cog-wheel valve. B and D show results from the same animals, but after the cog-wheel valve had been inactivated by application of acetylcholine.

View larger version (26K): [in a new window]   Fig. 3. Effect of cog-wheel valve contraction on pulmonary, right ventricular and systemic blood pressures in an alligator. (A) Pressure in the right ventricle (RV, red) and pulmonary artery (PA, green) and electrocardiograms (ECGs) from the centre of the right ventricle (RV ECG, red) and the cog-wheel valve muscle (cog-wheel ECG, blue) when the cog-wheel valve was functioning. (B) Pressures in the right ventricle (red) and systemic circulation (right aorta, RAo, cerise) and ECGs of the same animal with a functioning cog-wheel valve. (C) Same as A, except that the cog-wheel valve has been inactivated by injection with acetylcholine.

View larger version (14K): [in a new window]   Fig. 4. Delay between the electrocardiogram (ECG) in the right ventricle and the ECG in the cog-wheel valve muscle as a function of the distance between the two recording sites. One ECG electrode was left fixed in the middle of the cog-wheel valve muscle, while the electrode in the right ventricle was moved to different locations. Results from three animals are shown. Inverse slopes give the conduction velocity in the right ventricle, and are 0.43 m s-1 (P<0.001, r2=0.449), 1.41 m s-1 (P=0.14, r2=0.037) and 0.85 m s-1 (P<0.001, r2=0.152) from top to bottom, respectively. The intercept is the `nodal' delay at the junction of the right ventricle and cog-wheel valve muscle. Values are means ± S.E.M.

View larger version (14K): [in a new window]   Fig. 5. (A) Relationships between the delay separating the electrocardiogram (ECG) in the middle of the right ventricle from that in the cog-wheel valve muscle and heart rate (open circles) and between the phase of the cog-wheel muscle ECG and heart rate (filled circles): delay versus heart rate was not significant (P=0.24); phase versus heart rate slope=0.67 (P<0.001, r2=0.63). (B) Relationship between phase and the delay between the right ventricle and cog-wheel valve muscle ECGs: slope=0.069 (P<0.001, r2=0.58).