Predominant Distribution of Nifedipine-Insensitive, High Voltage–Activated Ca ²⁺ Channels in the Terminal Mesenteric Artery of Guinea Pig

Abstract

We have found nifedipine-insensitive (NI), rapidly inactivating, voltage-dependent Ca2+ channels (current, NI-I(Ca)) with unique biophysical and pharmacological properties in the terminal branches of guinea pig mesenteric artery, by using a whole-cell mode of the patch-clamp technique. The fraction of NI-I(Ca) appeared to increase dramatically along the lower branches of mesenteric artery, amounting to almost 100% of global I(Ca) in its periphery. With 5 mmol/L Ba2+ as the charge carrier, NI-I(Ca) was activated with a threshold of -50 mV, peaked at -10 mV, and was half-activated and inactivated at -11 and -52 mV, respectively, generating a potential range of constant activation near the resting membrane potential. The NI-I(Ca) was rundown resistant, was not subject to Ca(2+)-dependent inactivation, and exhibited the pore properties typical for high voltage-activated Ca2+ channels; Ba2+ is approximately 2-fold more permeable than Ca2+, and Cd2+ is a better blocker than Ni2+ (IC(50), 6 and 68 micromol/L, respectively). Relatively specific blockers for N- and P/Q-type Ca2+ channels such as omega-conotoxins GVIA and MVIIC (each 1 micromol/L) and omega-agatoxin IVA (1 micromol/L) were ineffective at inhibiting NI-I(Ca), whereas nimodipine partially (10 micromol/L; approximately 40%) and amiloride potently ( approximately 75% with 1 mmol/L; IC(50); 107 micromol/L) blocked the current. Although these properties are reminiscent of R-type Ca2+ channels, expression of the alpha(1E) mRNA was not detected using reverse transcriptase-polymerase chain reaction. These results strongly suggest the predominant presence of NI, high voltage-activated Ca2+ channels with novel properties, which may be abundantly expressed in peripheral small arterioles and contribute to their tone regulation.

Depolarization‐induced calcium influx in rat mesenteric small arterioles is mediated exclusively via mibefradil‐sensitive calcium channels

“…However, the biophysical properties of this channel were different from 'classical' low voltage-activated T-type channels (Morita et al, 1999). The major difference to T-type characteristics were activation at more depolarized potentials (threshold at À50 to À40 mV and peak current at À15 to À10 mV) than typical T-type currents with activation threshold at À70 to À60 mV and peak current at À40 to À30 mV.…”

“…In VSMC from rat or guinea-pig mesenteric artery, nifedipineinsensitive Ca 2 þ currents with T-type pharmacology activated at À50 to À40 mV and peaked at À10 to 0 mV (Morita et al, 1999;. In contrast, the three cloned T-type a 1 -subunits (Ca V 3.1, Ca V 3.2, and Ca V 3.3) all have an activation threshold at À70 mV and peak of I-V curve at À30 mV at physiological [Ca 2 þ ] (Perez- Reyes, 2003).…”

“…One possibility is through small window currents in the voltage range where steady-state activation and inactivation curves overlap (McDonald et al, 1994;Perez-Reyes, 2003). It has been reported that a T-type window current was responsible for a physiological, steady-state Ca 2 þ entry in human myoblasts (Bijlenga et al, 2000), and window currents in the V m range from À50 to À20 mV were suggested to account for high-K þ -induced, nifedipine-insensitive Ca 2 þ entry and vasoconstriction in rat mesenteric artery (Morita et al, 1999;Itonaga et al, 2002;Inoue & Mori, 2003). In pressurized small arterioles (hamster cheek pouch) resting V m measured in vivo was À35 mV (Welsh & Segal, 1998).…”

“…Moreover, physical ion pore properties (Ba 2 þ /Ca 2 þ permeability; Cd 2 þ /Ni 2 þ blocking efficacy) suggested that this channel belongs to the class of high-voltage activated Ca 2 þ channels (e.g. R-type), rather than low-voltage activated (T-type) channels (Morita et al, 1999;. These authors therefore suggested that this channel is either a novel type of voltage-activated Ca 2 þ channel with mixed properties known from R-and T-type channels or an alternative splice variant of the T-type channel.…”

“…However, small steady-state Ca 2 þ currents occurring in the voltage 'window' between overlapping steady-state activation and inactivation curves (window currents) may account for sustained Ca 2 þ influx into cells (McDonald et al, 1994;Morita et al, 1999;Perez-Reyes, 2003). A physiological role for T-type window currents has recently been demonstrated in human myoblast terminal differentiation where they mediated the sustained Ca 2 þ entry required for myoblast fusion (Bijlenga et al, 2000).…”