World Congress of Malacology Antwerp ... - Unitas Malacologica

Heterometric autoregulation in the bivalve heart
Collis, Leon 1 ; Sun, Ying 2 ; Hill, Robert B. 3
1: BioCurrents Research Center, Marine Biological Laboratory, Woods Hole, MA, USA,
Email: lcollis@mbl.edu
2: Dept. of Electrical and Computer and Biomedical Engineering, University of Rhode Island,
Kingston, RI, USA,
Email: sun@ele.uri.edu
3: Dept. of Biological Sciences, University of Rhode Island, Kingston, RI, USA,
Email: bob@uri.edu
Straub (early 1900's) was first to identify heterometric autoregulation in the molluscan heart. This
was prior to the establishment of the Frank-Starling Law of the heart (1910-1930). It is now evident
that the relationship of force and stretch of the cardiac myofibrils is dependent on the mobilization of
calcium and to the sensitivity of the contractile apparatus to calcium. Despite the relative abundance
of mammalian studies, there is relatively little information on the effect of stretch and rapid
shortening on tension in the invertebrate heart. Using digital control, we applied rapid length
perturbations to cardiac ventricular trabeculae of the bivalve, Spisula solidissima (Heterodonta,
Mactridae). Trabeculae lacked a pronounced descending limb of the force-length relationship
supporting the notion that molluscan cardiac muscle is able to tolerate large fluctuations in length.
We consequently identified and characterized length-dependent activation and deactivation of force
during contractile activity in the bivalve heart. The magnitude of deactivation was dependent on the
size of the shortening event and the point at which shortening was applied during the cycle of
contraction. Our pharmacological data suggests that a reduction in calcium at the contractile element
and/or sequestration of calcium may occur during shortening of the muscle. Electron microscopy
revealed ultrastructure that resembled mammalian smooth muscle, and possible mechanisms of
deactivation may be similar to those observed in this muscle type. Deactivation may minimize the
magnitude of work done during active shortening of bivalve cardiac muscle, particularly against the
low afterload exhibited in the bivalve peripheral circulatory system. Changes in intracellular calcium
during sudden length perturbations may explain the effect of stretch on action potential duration in
the bivalve heart, as shown by previous investigators.
Patchy distribution of the (TTAGGG)n telomeric repeat motif in Mollusca
Colomba, Mariastella 1 ; Libertini, Angelo 2 ; Gregorini, Armando 1 ; Vitturi, Roberto 3
1. University of Urbino, Via Maggetti, 22, 61029 Urbino, Italy,
Email: m.colomba@uniurb.it; a.gregorini@uniurb.it
2. Institute of Marine Biology, CNR, Riva 7 Martiri 1364/a, 30122 Venezia, Italy
3. University of Palermo, via Archirafi 18, 90123 Palermo, Italy
Telomeric sequences are thought to be constant in large taxonomic groups. Vertebrates display a
hexamer repeat motif, (TTAGGG)n, which is conserved in all species so far examined from mammals
to fish. On the contrary, invertebrates – being a polyphyletic group - exhibit a certain degree of
heterogeneity. In fact, although the vertebrate-type (TTAGGG)n is common, at least two other
slightly different motifs as (TTAGG)n and (TTAGGC)n have been reported. With reference to the
species of molluscs analysed by our group, only in two of them, Oxynoe olivacea (Opisthobranchia,
Sacoglossa) and Crepidula unguiformis (Prosobranchia, Caenogastropoda) telomeres do not
hybridize to (TTAGGG)n and, at present, their sequences remain unidentified. This findings provides
evidence supporting a patchy distribution of telomeric repeats within Mollusca.
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