Meeting Abstract

P2.111A  Jan. 5  Embryonic larval heart and cardiac response to hypoxia in a prosobanch mollusc BYRNE, R.A.*; RUNDLE, S.D.; SPICER, J.I.; SUNY Fredonia, US; Univ. of Plymouth, UK; Univ. of Plymouth, UK Roger.Byrne@fredonia.edu

Embryos of the prosobranch gastropod, Bithynia tentaculata, develop a temporary larval �heart� (LH), an accessory pulsatile structure prior to development of a true heart. Both structures are present during an intermediate stage, while the LH ceases function and disappears in later stages as the heart completes its development. LH function is uncertain but may distribute hemolymph and aid in gas exchange prior to ontogeny of a functional circulatory system. We examined changes in beat frequency in LH and true heart during five sequential embryonic stages at 18�C. Mean LH frequency was 24.4�1.5 (SD) during earlier stages rising to 32.5�4.5 at the onset of true heart function and declining thereafter to 20.1�3.7, before the organ ceased to function. Heart beat frequency was 36.1�9.2 at the onset of cardiac activity and increased to 50.1�6.8 after disappearance of the LH. Peak LH frequency is coincident with early cardiac development and declines once the CV system becomes more functional. We examined the response of both LH and true hearts to severe hypoxia over three developmental stages: embryos with LH only, those with both LH and true hearts, and a later stage with only true hearts. In all cases, mean LH frequency declined steadily in response to hypoxic treatment reaching ~20% of normoxic control rate after 60 min., suggesting a gradual reduction in energy utilization as metabolism was depressed in response to hypoxia. In contrast, mean cardiac frequency declined almost immediately to <10% of normoxic control and remained at this level for the 60 min hypoxic treatment, indicative of a more centrally regulated response to hypoxia. Sensitivity of the cardiac response was more evident in the later more developed stage. Our study suggests that the embryonic cardiac systems are under neural control from the earliest stage and demonstrate a severe bradycardia in response to hypoxia, whereas temporary LH activity is related to general metabolic rates.