Effects of 2,3‐butanedione monoxime on whole‐cell Ca2+ channel currents in single cells of the guinea‐pig taenia caeci.

Abstract

SUMMARY1. The inhibitory actions of cadmium (Cd2+), nifedipine and 2,3-butanedione monoxime (BDM) on whole-cell Ca21 channel currents in single cells of the guinea-pig taenia caeci were investigated using a single-electrode whole-cell voltage-clamp technique.2. Calcium channel currents were isolated using pipette solutions containing Cs+, tetraethylammonium and ATP (3 mM). Ca2+ or Ba2+ (7-5 mM) in the bathing solution acted as the charge carrier during inward current flow. Ca2+ channel currents in 7.5 mM-Ba2+ (IBa) were recorded at potentials positive to -40 mV, were maximal near 0 mV and reversed near + 60 mV. Ca2+ channel activation showed a sigmoidal relationship with potential, which was half-maximal at -13 mV.3. Both the inward and outward flow of current was depressed and eventually blocked by 0 3-100 ,tM-Cd2+, 0 1-10 ftM-nifedipine and 2-20 mm-BDM. Half-maximal blockade of IBa at 0 mV was achieved with -3 /tM-Cd2 , 1 /LM-nifedipine and 10 mM-BDM. Steady-state activation curves were not affected by Cd2+ or BDM, but were shifted in the hyperpolarizing direction by nifedipine at concentrations > 1 /,M.4. Calcium channel currents in single cells and K+ contractures in intact strips were both blocked in a voltage-dependent manner. Steady-state inactivation curves (f.o(V)) for IBa were shifted 20 mV in the hyperpolarizing direction by 0-3 /iMnifedipine and 4 mV by 10 mM-BDM. From these shifts a dissociation binding constant to inactivated Ca2+ channels for nifedipine was estimated as 78 nm, and for BDM, 5 mM.5. At 10/M Cd2+ produced a 43 + 6 % (n = 3) block of the inward current at 0 mV when Ca2+ (7 5

Effects of okadaic acid and ATPγS on cell length and Ca2+‐channel currents recorded in single smooth muscle cells of the guinea‐pig taenia caeci

“…Addition of ATPyS (1 mM) to the pipette solution decreased the initial amplitude of 'Ba recorded as soon as was practical after membrane rupture. The averaged peak amplitude (I(peak) of IBa in cells filled with 1 mm ATPyS: 3mM ATP was 134 + 19 (n = 13) pA, significantly smaller than control cells containing 4 mM ATP (209 + 24 (n = 5) pA; P < 0.05) (Lang & Paul, 1991).…”

“…Both the inward and outward flow of current was blocked by 100puM Cd (Figure la,b). (Langton et al, 1989;Lang & Paul, 1991). The later decline in amplitude is thought to arise from Ca2+ channel rundown, the rate of which was variable between cells (Lang & Paul, 1991 ATPyS or okadaic acid.…”

“…(Langton et al, 1989;Lang & Paul, 1991). The later decline in amplitude is thought to arise from Ca2+ channel rundown, the rate of which was variable between cells (Lang & Paul, 1991 ATPyS or okadaic acid. ATPyS donates thiophosphate groups to cell proteins, which are resistant to enzymatic hydrolysis (Gratecos & Fischer, 1974).…”

“…The fundamental properties of the whole-cell Ca2+ channel currents recorded in single cells from the guinea-pig taenia caeci bathed in 7.5 mm Ba2+, have been described recently (Ganitkevich et al, 1986;Lang & Paul, 1991) and are summarized in Figure 1. All cells were held at -80 mV.…”

“…Whole-cell currents were recorded by standard patch-clamp techniques (Hamill et al, 1981) and low-resistance pipettes (2-7 MQ), as described recently (Lang & Paul, 1991). The initial formation of the cell-attached patch and rupture of the underlying cell membrane to create the whole-cell voltage clamp mode were performed in Ca2 '-containing (1.5 mm Ca2 +) saline (see below).…”