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Calcium regulation of the contractile apparatus in canine gastric smooth muscle


, : Calcium regulation of the contractile apparatus in canine gastric smooth muscle. Journal of Physiology (Cambridge) 460(0): 33-50

The relationships between cytosolic Ca-2+ ((Ca-2+)-cyt; expressed as a fluorescence ratio at 400 nm and 500 nm using Indo-1) and contractile force was examined in strips of circular smooth muscles of canine gastric antrum. Rhythmic increases in (Ca-2+)-cyt were observed and contractions were biphasic. In most muscles (70%), the amplitude of the second phase of the Ca-2+ transient was less than or equal to the first phase of the Ca-2+ transient, but the second phase of the contraction was much smaller than the first phase, suggesting a decrease in Ca-2+ sensitivity during the second contractile phase. In 30% of muscles, the amplitude of the second phase of the Ca-2+ transient was 2- to 3-fold greater than the first phase. In these muscles, the second phase of contraction was 10-fold greater than the first phase of contraction. Thus, a non-linear relationship between (Ca-2+)-cyt and force greatly amplifies force development when (Ca-2+)-cyt exceeds a threshold level. Acetylcholine (ACh, 0.3-1 mu-M) increased the amplitudes of Ca-2+ transients and basal (Ca-2+)-cyt between phasic contractions. The increase in basal (Ca-2+)-cyt did not cause tone to develop. ACh increased the amplitude of Ca-2+ transients 2- or 3-fold and this was associated with a 15 to 20-fold increase in the force of phasic contractions. Pentagastrin (0.5 nM) and cholecystokinin octapeptide (CCK, 40 nM) had similar effects on Ca-2+ transients and phasic contractions. Bay K 8644 (0.1 mu-M) and TEA (5 mM) also increased the amplitudes of Ca-2+ transients by 2- to 3-fold and phasic contractions by 15- to 30-fold. There was no significant difference observed between the (Ca-2+)-cyt-force relationships in the presence of agonists (i.e. ACh, pentagastrin and CCK) or when (Ca-2+)-cyt was increased by Bay K 8644 or TEA. These data suggest that agonist-dependent increases in Ca-2+ sensitivity may not significantly regulate the (Ca-2+)-cyt-force relationship in antral muscles. D600 (5 mu-M), added during stimulation with ACh (0.3 M), decreased (Ca-2+)-cyt and force without affecting the (Ca-2+)-cyt-force relationship. Mechanisms exist for agonist-mediated enhancement of the Ca-2+-force relationship. In alpha-toxin-permeabilized antrum, ACh (10 mu-M) with GTP (100 mu-M) or GTP-gamma-S (100 mu-M) increased the Ca-2+-induced contraction at clamped levels of Ca-2+. Phorbol 12,13-dibutyrate (PDBu, 10 mu-M) also increased the contractile force at a given level of Ca-2+. These data suggest that agonists that increase phosphatidyl inositol turnover might enhance Ca-2+ sensitivity, but this effect was not substantial at physiological levels of Ca-2+ in these muscles. Addition of PDBu (0.5 mu-M) to intact muscles reduced Ca-2+ transients and contractions, suggesting that activation of C kinase may have inhibitory effects on Ca-2+ influx. These results suggest that there is a very steep relationship between (Ca-2+)-cyt and force in antral muscles. Although the muscle possesses agonist-dependent mechanisms to increase Ca-2+ sensitivity, it appears that these systems do not participate in agonist-induced contractions. This phasic muscle appears to be already primed to produce forceful contractions with modest increases in (Ca-2+)-cyt.

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