Demonstration of an “Excitation-Contraction Recoupling” Mechanism in Mammalian Ventricular Myocardium

A PROCESS called “excitation-contraction coupling” has been generally accepted to take place only in the direction of excitation to contraction. Through this mechanism a propagated action potential initiates an active state in skeletal or cardiac muscle and the muscle contracts. We propose that, in the mammalian ventricular myocardium at least, the process is not unidirectional and an important reverse coupling between the contractile system and the excitable plasma membrane has been overlooked. Through this feedback interaction the mode of contraction (that is, isotonic or isometric) not only determines the instantaneous electrical state of the plasma membrane, but also influences the mechanical events of the subsequent beats. Thus when Kaufmann et al.¹ recorded intracellular action potentials from cat papillary muscle, the time course of the repolarization was altered depending on the mode of contraction. Some kind of contraction-excitation feedback has also been suggested by Stauch² and Lab^3,4. They showed a difference in the shape of the monophasic action potential, as recorded by a suction electrode, when comparing isotonic and isovolumic contraction of the intact ventricle. But their experimental conditions did not allow satisfactory analysis of the phenomenon.     

Maternal Effects on Offspring Quality in Poeciliid Fishes

SYNOPSIS. We evaluated the effects of maternal environment onoffspring size and composition in three species of poeciliidfishes. We chose food availability as the environmental factorfor study. Mature females were assigned to either high or lowfood for an interval of time, then randomly reassigned to highor low food, with the restriction that there be equal numbersin each of four treatments: high-high, high-low, lowhigh, andlow-low food availability. The three species chosen for studydiffer in the pattern of maternal provisioning. Poecilia reticulataand Priapichthys festae mothers provide all resources necessaryfor development as yolk, prior to fertilization. In contrast,Heterandria formosa mothers continue to provision the youngthroughout development. These species also differ in whetheror not they have superfetation, or the ability to carry multiplebroods of young in different stages of development. P. reticulatadoes not have superfetation while the other two species do.We were interested in whether the pattern of maternal provisioningor superfetation influenced the maternal effect. The two lecithotrophicspecies responded to low food by producing larger young withgreater fat reserves. H. formosa, the matrotrophic species,responded to low food by producing smaller young. We proposethat the production of large young in the face of low food availabilitymight represent adaptive plasticity; matrotrophy might representa constraint that prevents such an adaptive response. Superfetationhad no impact on this maternal effect.     

Cymbonotus (Compositae: Arctotideae, Arctotidinae): an endemic Australian genus embedded in a southern African clade

The Compositae (Asteraceae) is the largest flowering plant family if described, accepted taxa are considered. Recent revisions in the taxonomy of the family have resulted in the recognition of ten subfamilies and 35 tribes. The tribe Arctotideae is one of the smallest, with around 200 species; it contains two subtribes and several hard-to-place taxa. Previous work has shown that the subtribe Arctotidinae is well defined and is restricted to southern Africa, except for the Australian genus Cymbonotus . Molecular data from internal transcribed spacer (ITS), ndh F, and trn L-F sequences were used (24 previously published sequences; 47 new sequences) to determine the patterns of relationships within the subtribe. Twenty-three samples from the ingroup, including members of all genera and all three species of Cymbonotus , were included in the analysis, together with two outgroup taxa. Cymbonotus is monophyletic and deeply embedded in the subtribe; Haplocarpha is paraphyletic and basal in position; all other genera are monophyletic; however, Arctotis has over 60 species and only eight were sampled for this study, so additional work may prove otherwise. Arctotis is nested high in the tree and has short branch lengths; this may reflect recent radiation. By contrast, the species of the paraphyletic and basal Haplocarpha have long branches, which may indicate an older radiation and a shared ancestry with the remainder of the subtribe. The presence of Cymbonotus in Australia is most probably the result of long-distance dispersal.  Journal compilation © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society , 2007, 153 , 1–8. No claim to original US government works