The sources and sequestration of Ca²⁺ contributing to neuroeffector Ca²⁺ transients in the mouse vas deferens

Abstract

The detection of focal Ca2+ transients (called neuroeffector Ca2+ transients, or NCTs) in smooth muscle of the mouse isolated vas deferens has been used to detect the packeted release of ATP from nerve terminal varicosities acting at postjunctional P2X receptors. The present study investigates the sources and sequestration of Ca2+ in NCTs. Smooth muscle cells in whole mouse deferens were loaded with the Ca2+ indicator Oregon Green 488 BAPTA‐1 AM and viewed with a confocal microscope. Ryanodine (10 µm) decreased the amplitude of NCTs by 45 ± 6 %. Cyclopiazonic acid slowed the recovery of NCTs (from a time course of 200 ± 10 ms to 800 ± 100 ms). Caffeine (3 mm) induced spontaneous focal smooth muscle Ca2+ transients (sparks). Neither of the T‐type Ca2+ channel blockers NiCl2 (50 µm) or mibefradil dihydrochloride (10 µm) affected the amplitude of excitatory junction potentials (2 ± 5 % and −3 ± 10 %) or NCTs (−20 ± 36 % and 3 ± 13 %). In about 20 % of cells, NCTs were associated with a local, subcellular twitch that remained in the presence of the α1‐adrenoceptor antagonist prazosin (100 nm), showing that NCTs can initiate local contractions. Slow (5.8 ± 0.4 µm s−1), spontaneous smooth muscle Ca2+ waves were occasionally observed. Thus, Ca2+ stores initially amplify and then sequester the Ca2+ that enters through P2X receptors and there is no amplification by local voltage‐gated Ca2+ channels.

T-type Ca2+ channels in non-vascular smooth muscles

“…A role for transmembrane Ca 2+ influx has been implicated in this overall scheme, but the route is not known. Both Ni 2+ (50 µM) and mibefradil were ineffective in attenuating EJPs or intracellular Ca 2+ transients, suggesting that the T-type channel is not involved [67,68]. This contrasts to a study with mesenteric blood vessels, which showed the Ni 2+ did attenuate EJPs [68] and shows that similar phenomena are regulated by alternative processes in different cells.…”

A possible role of the cholinergic and purinergic receptor interaction in the regulation of the rat urinary bladder function

“…Field stimulation of the smooth muscle evoked local, transient Ca 2+ release events. These events were mediated by postsynaptic smooth muscle P2X receptors activated by ATP released from synaptic varicosities (Brain et al 2002(Brain et al , 2003. Pharmacological studies also confirmed that junctional Ca 2+ transients were mediated through the activation of purinergic receptors (Lamont and Wier, 2002;Lamont et al 2003).…”

Calcium Cycling in Synthetic and Contractile Phasic or Tonic Vascular Smooth Muscle Cells

“…The initial release occurs in the vicinity of the plasma membrane. It spreads into the cell through the regenerative release of Ca 2+ by the RyR and /or the IP3R in the form of an intracellular Ca 2+ wave travelling down the length of the cell [31][32][33]. When [Ca 2+ ]i is integrated over an entire cell with time, these Ca 2+ waves appear as rhythmical oscillations [34].…”