Spontaneous transient depolarizations in lymphatic vessels of the guinea pig mesentery: pharmacology and implication for spontaneous contractility

von der Weid PY, Rahman M, Imtiaz MS, van Helden DF. Spontaneous transient depolarizations in lymphatic vessels of the guinea pig mesentery: pharmacology and implication for spontaneous contractility. Am J Physiol Heart Circ Physiol 295: H1989-H2000, 2008; doi:10.1152/ajpheart.00007.2008. - Guinea pig mesenteric lymphatic vessels exhibit rhythmic constrictions induced by action potential (AP)-like spikes and initiated by entrainment of spontaneous transient depolarizations (STDs). To characterize STDs and the signaling mechanisms responsible for their occurrence, we used intracellular microelectrodes, Ca(2+) imaging, and pharmacological agents. In our investigation of the role of intracellular Ca(2+) released from Ca(2+) stores, we observed that intracellular Ca(2+) transients accompanied some STDs, although there were many exceptions where Ca(2+) transients occurred without accompanying STDs. STD frequency and amplitude were markedly affected by activators/inhibitors of inositol 1,4,5-trisphosphate receptors (IP(3)Rs) but not by treatments known to alter Ca(2+) release via ryanodine receptors. A role for Ca(2+)-activated Cl(-) (Cl(Ca)) channels was indicated, as STDs were dependent on the Cl(-) but not Na(+) concentration of the superfusing solution and were inhibited by the ClCa channel blockers niflumic acid (NFA), anthracene 9-carboxylic acid, and 5-nitro-2-(3-phenylpropylamino) benzoic acid but not by the volume-regulated Cl(-) blocker DIDS. Increases in STD frequency and amplitude induced by agonist stimulation were also inhibited by NFA. Nifedipine, the hyperpolarization- activated inward current blocker ZD-7288, and the nonselective cation/storeoperated channel blockers SKF-96365, Gd(3+), and Ni(2+) had no or marginal effects on STD activity. However, nifedipine, 2-aminoethoxydiphenyl borate, NFA, SKF-96365, Gd(3+), and Ni(2+) altered the occurrence of spontaneous APs. Our findings support a role for Ca(2+) release through IP(3)Rs and a resultant opening of ClCa channels in STD generation and confirm the importance of these events in the initiation of lymphatic spontaneous APs and subsequent contractions. The abolition of spontaneous APs by blockers of other excitatory ion channels suggests a contribution of these conductances to lymphatic pacemaking.