Nitrergic relaxation in rat gastric fundus: influence of mechanism of induced tone and possible role of sarcoplasmic/endoplasmic reticulum Ca 2+ ATPase

Abstract

The aim of this study was to investigate the influence of the mechanism of induced tone and the role of sarcoplasmic/endoplasmic reticulum Ca 2+ ATPase (SERCA) in nitrergic relaxation of rat gastric fundus. Prostaglandin F 2α (PGF 2α), thapsigargin (TSG) and cyclopiazonic acid (CPA) were used in concentrations that induced a similar contraction (20 g force/g tissue). Nifedipine (3 × 10 −7 M) completely relaxed PGF 2α-contracted tissues and relaxed tissues contracted by TSG and CPA by 20 ± 6% and 56 ± 12% respectively; contraction induced by the three contractile agents was fully reversed by a general Ca 2+ entry blocker 1-[2-(4-methoxyphenyl)-2-[3-(4-metoxyphenyl)propoxy]ethyl-1H-imidazole HCl (SKF 96365; 10 −5 M). In the presence of nifedipine (3 × 10 −7 M) or verapamil (10 −5 M), PGF 2α and CPA-induced contractions were still approximately 50% relaxed by SKF 96365. This suggests that contractions induced by PGF 2α are related to Ca 2+ entry through L-type voltage-operated Ca 2+ channels and that contractions by TSG are mainly related to Ca 2+ entry through store-operated Ca 2+ channels. Relaxant responses to exogenous nitric oxide (NO), to endogenous NO released by electrical field stimulation, and to vasoactive intestinal polypeptide (VIP) were studied in tissues contracted by TSG and CPA and compared to responses in tissues contracted by PGF 2α. Responses to exogenous and endogenous NO were greatly attenuated in TSG-contracted tissues, but not in CPA-contracted tissues. When contraction was induced by CPA in the presence of nifedipine or verapamil, relaxations to exogenous and endogenous NO were also significantly reduced. Relaxation induced by VIP was reduced in tissues contracted by either TSG or CPA in the presence of nifedipine or verapamil. These results suggest that the ability of the nitrergic neurotransmitter to induce relaxation of rat gastric fundus is influenced by the mechanism used to induce tone and are indicative for a role for SERCA in nitrergic relaxation. However, activation of SERCA appears to not be unique for nitrergic relaxation, but might also be used by VIP, a co-transmitter of NO in this tissue.