The effects of manganese and barium on the cardiac pacemaker current,i f,in rabbit sino-atrial node myocytes

Summary The isolation of ionic fluxes contributing to electric currents through cell membranes often requires block of other undesired components which can be achieved, among others, by divalent cations. Mn²⁺ and Ba²⁺ are often used, for example, to block Ca and K currents. Here we have investigated the effects of these two cations on the properties of the hyperpolarization-activated pacemaker current i_f, in rabbit sino-atrial node myocytes, as obtained by voltage clamp analysis. We find that 2 mM Mn²⁺ shifts the i_f activation curve by 3.2±0.3 mV towards more positive values. However, when 1 mM Ba²⁺ is also added, the positive shift is more than halved (1.3±0.2 mV). We find, too, that in the absence of blocking cations the ACh-induced i_f inhibition is slightly higher than in their presence. These results indicate that the alteration of i_f kinetic properties by Ba²⁺ plus Mn²⁺-containing solutions is minimal.     

dNTP pools imbalance as a signal to initiate apoptosis

Fidelity in DNA synthesis and repair is largely dependent on a balanced supply of deoxynucleotide triphosphate (dNTP) pools. Results from different groups have shown that alterations in dNTP supply result in DNA fragmentation and cell death with characteristics of apoptosis. We have recently shown that in apoptosis driven by deprivation of interleukin-3 (IL-3) in a murine hemopoietic cell line, there is a rapid imbalance in the availability of dNTP that precedes DNA fragmentation. In these cells, dNTP pool balance is closely coupled to the function of the salvage pathway of dNTP synthesis. Apoptosis, induced by treatment of these cells with drugs that inhibit the de novo dNTP synthesis, is prevented when dNTP precursors are supplied through the salvage pathway. IL-3 regulates thymidine kinase activity, suggesting that alterations in dNTP metabolism after IL-3 deprivation could be a relevant event in the commitment of hemopoietic cells to apoptosis.     

Heterochromatin characterization and distribution in the chromosomes of two populations ofIdotea baltica basteri Audouin, 1826 (Isopoda,Valvifera)

Two mediterranean populations ofIdotea baltica basteri from Messina and Naples showed a set of chromosomes composed by 58 all-biarmed chromosomes. The heterochromatin was located in the pericentromeric region of the chromosomes, and its composition appeared heterogeneous. In fact, not all the homologs showed heterochromatin resistant to digestion with three restriction enzymes (Alu I, Hae III and Sau 3A). Moreover, the two populations showed polymorphism in a band of G+C-rich telomeric heterochromatin, which was present only in the population from Messina. It is hypothesized that chromosomal polymorphism might reflect the geographical isolation of the two populations. It is also suggested that a process of diversification is taking place.     

Neuronal cell-cell adhesion depends on interactions of N-CAM with heparin-like molecules

Cell-cell interactions are of critical importance during neural development, particularly since the migration of neural cells and the establishment of functional interactions between growing axons and their target cells has been suggested to depend upon cell recognition processes. Neurone-neurone adhesion has been well studied in vitro, and is mediated in part by the neural cell adhesion molecule N-CAM. N-CAM-mediated cell-cell adhesion has been postulated to occur by a homophilic binding mechanism, in which N-CAM on the surface of one cell binds to N-CAM on a neighbouring cell. Studies in our laboratory have identified a cell surface glycoprotein, now known to be N-CAM, which participates in cell-substratum interactions in the developing chicken nervous system. Although this adhesion involves a homophilic binding mechanism, the binding of the cell surface proteoglycan heparan sulphate to the glycoprotein is also required. This raises the question of whether the binding of heparan sulphate to N-CAM is also required for cell-cell adhesion. Here we show that the binding of retinal probe cells to retinal cell monolayers is inhibited by heparin, a functional analogue of heparan sulphate, but not by chondroitin sulphate. Monoclonal antibodies that recognize two different domains on N-CAM, the homophilic-binding and heparin-binding domains, inhibit cell-cell adhesion. The heparin-binding domain isolated from N-CAM by selective proteolysis also inhibits cell-cell adhesion when bound to the probe cells.