Functional and morphological organization of the cat sinoatrial node

The feline sinoatrial node has a unifocal impulse generation as previously described for rodents. Its main component is collagen. The primary pacemaker consists of at most 2000 cells, but appears to function normally with less than 500 cells. Primary pacemaker cells are found in the area where empty cells are predominant. A negative correlation between myofilament density and diastolic depolarization rate, known to exist in the rabbit and guinea-pig, is absent in the cat. Gap junctions are seen in the center and in the periphery of the nodal region, but they are extremely rare. The electrophysiological characteristics of the primary pacemaker of the cat are quite similar to those of the rabbit, although the nodal morphology is very different. Abrupt transitions from one cell type into another are observed in the feline sinoatrial node. From this morphological point of view the feline sinoatrial node resembles the canine and human sinoatrial nodes more than the lapine sinoatrial nod

A molecular perspective of the genetic relationships of G-protein coupled melatonin receptor subtypes

Successful cloning of melatonin receptors from various target tissues in the past few years has increased our understanding of the molecular signal transduction mechanisms of G-protein coupled melatonin receptors, of which three subtypes (MEL-1A, MEL-1B, and MEL-1C) have been reported in different vertebrates. Based upon melatonin receptor sequences available in the Genbank database, we have performed phylogenetic analyses of the nucleotide and encoded amino acid sequences of G-protein-coupled melatonin receptors, and determined the range of amino acid identities between melatonin receptors of the same and different subtypes. Besides the three well-known subtypes, a potential novel subtype of MEL-1D, as exemplified by unique separation of Xenopus X2.0 sequence (Genbank accession No. U31826) from the others in the protein phylogenetic tree, possibly exists. In addition, one of the chicken brain melatonin receptor sequences has been identified as belonging to the MEL-1B subtype. Our analyses showed that melatonin receptors of the same subtype and different subtypes are likely to share ≥75% and <65% amino acid identities, respectively. Phylogenetic analysis based on amino acid comparisons will be needed to determine the subtype status of any pair of melatonin receptor sequences that exhibit ≥65% to <75% amino acid identity. Despite the usefulness of genetic relatedness in the subtype classification of G-protein-coupled melatonin receptors, functional correlation of molecular structure may ultimately prove the most comprehensive approach in melatonin receptor classification. © Munksgaard.link_to_subscribed_fulltex