Induction and organization of Ca2+ waves by enteric neural reflexes.

The motility of the gastrointestinal tract consists of local, non-propulsive mixing (pendular or segmental) and propulsive (peristaltic) movements. It is generally considered that mixing movements are produced by intrinsic pacemakers which generate rhythmic contractions, and peristalsis by intrinsic excitatory and inhibitory neural reflex pathways, but the relationship between mixing and peristalsis is poorly understood. Peristalsis is compromised in mice lacking interstitial cells of Cajal, suggesting that these pacemaker cells may also be involved in neural reflexes. Here we show that mixing movements within longitudinal muscle result from spontaneously generated waves of elevated internal calcium concentration which originate from discrete locations (pacing sites), spread with anisotropic conduction velocities in al directions, and terminate by colliding with each other or with adjacent neurally suppressed regions. Excitatory neural reflexes control the spread of excitability by inducing new pacing sites and enhancing the overall frequency of pacing, whereas inhibitory reflexes suppress the ability of calcium waves to propagate. We provide evidence that the enteric nervous system organizes mixing movements to generate peristalsis, linking the neural regulation of pacemakers to both types of gut motility.     

Mechanical and biochemical characterization of the contraction elicited by a calcium-independent myosin light chain kinase in chemically skinned smooth muscle

Summary The contraction induced by a Ca²⁺-independent myosin light chain kinase (MLCK-) was characterized in terms of isometric force (F_o), immediate elastic recoil (SE), unloaded shortening velocity (V_us), shortening under a constant load and ATPase activity of chemically skinned smooth muscle preparations. These parameters were compared to those measured in a Ca²⁺-induced contraction to assess the nature of cross bridge interaction in the MLCK-induced contraction. F_o developed in chicken gizzard fibers as well as SE were similar in contractions elicited by either agent. V_us in the contraction induced by MLCK-(0.36 mg/ml) was similar though averaged 39.3±8.9% less than V_us induced by Ca²⁺ (1.6x10^–6M) in the control fibers. Addition of Ca²⁺ (1.6x10^–6M) to a contraction induced by MLCK-resulted in small increases in both F_o and V_us. Shortening under a constant load was similar for both types of contractions. The contraction induced by MLCK-was accompanied by an increased rate of ATP hydrolysis. The MLCK-induced contraction is thus kinetically similar though not identical to a contraction induced by Ca²⁺. We conclude that with respect to actin-myosin interaction, MLCK- and Ca²⁺-induced contractions are similar.     

Effect of testosterone on the kinetics of the development of suppressor cells in adjuvant arthritis

Summary The induction of unresponsiveness to mycobacterial adjuvant took a longer time in male DA rats than in female rats. A shift in the induction time of unresponsiveness in males toward the female type was brought about by castration, but could be reverted to the male type by the application of testosterone. The transfer study revealed that cells capable of preventing arthritis required a longer incubation time for their development in males than in females. This suggests that testosterone inhibits the development of suppressor cells in adjuvant arthritis.Acknowledgments. We are grateful to The Naito Foundation Research Grant for 1983.     

Velocity and myosin phosphorylation transients in arterial smooth muscle: effects of agonist diffusion

Summary Transients in myoplasmic [Ca²⁺] and in phosphorylation of the 20,000 dalton light chain of myosin have been reported following stimulation of vascular smooth muscle by various agonists. Since these transients are rapid compared with the time required to attain a steady-state stress, agonist diffusion rates may be a significant limitation in activation. The purpose of this study was to estimate the effect of agonist diffusion rates on the time course of activation as assessed by mechanical measurements of stress development and isotonic shortening velocities and by determinations of the time course of myosin phosphorylation. The approach was to measure these parameters in K⁺-stimulated preparations of the swine carotid media of varying thicknesses and to estimate the theoretical contributions imposed by diffusion rates and the presence of a diffusion boundary layer surrounding the tissue. The results show that the time course of parameters which are tissue averages such as stiffness, active stress, and myosin phosphorylation is dominated by agonist diffusion rates. The sequence of events involved in excitation-contraction coupling including agonist actions on the cell membrane, Ca²⁺ release, activation of myosin light chain kinase, and cross-bridge phosphorylation appear to be very rapid events compared with stress development. Estimates of unloaded or lightly loaded shortening velocities which are not simple tissue averages appear to provide an imporoved estimate of activation rates.Supported by a grant from the National Institutes of Health 5-PO1-HL19242. K. E. Kamm was supported by a National Heart, Lung, and Blood Institute Research Service Award HL-05957.     

Enhanced expression of Epstein-Barr virus early antigens by antitubulin agents in a latently infected human lymphoblastoid cell line

28 anticancer agents have been surveyed for Epstein-Barr virus (EBV) activating potency. Two vinca alkaloids with antitubulin activity, vinblastine (VLB) and vincristine (VCR), enhanced the expression of EBV early antigens (EA) in a latently infected human lymphoblastoid cell line (Raji), when used in combination with n-butyrate. Other antitubulin agents, colchicine, colcemid, and podophyllotoxin, had the same effect, although their effects were less than that of the potent tumor promoter, 12-O-tetradecanoyl phorbol-13-acetate (TPA).     

Mitogens trigger a calcium-independent signal for proliferation in phorbol-ester-treated lymphocytes

The activation of T lymphocytes by mitogens requires at least two signals; the first, delivered to T cells by a mitogen in conjunction with accessory cells (monocytes/macrophages), leads to the generation of the second signal, interleukin-2 (IL-2). The first signal also induces the expression of IL-2 receptors on the surface of a subpopulation of T cells; binding of IL-2 to its receptor then initiates a cascade of events culminating in DNA synthesis by these cells. Certain compounds act synergistically with mitogens in promoting T-cell proliferation by substituting for the activities of interacting cells or their products. For example, the phorbol ester 12-O-tetradecanoyl phorbol 13-acetate (TPA) has been shown to restore the ability of macrophage-depleted T-cell populations to respond to mitogenic lectins. Transmembrane fluxes of calcium, leading to increased free cytosolic calcium concentrations ([Ca2+]), have been demonstrated following mitogen binding to lymphocytes and have been implicated in the initiation of cell proliferation. We show here that the effect of TPA on lymphocyte proliferation occurs in the absence of extracellular Ca2+ or detectable changes in [Ca2+]i, but only in the presence of mitogens. This suggests that in cells which have been incubated with the phorbol ester, mitogens can induce proliferation by a calcium-independent signal.