In the present study the effects of PKC activation and inhibition on Ca²⁺ transients and phasic contractions evoked by electrical field stimulation (EFS) in vitro in ureteric smooth muscle have been investigated. Simultaneous measurements of Ca²⁺ and force was done using Indo-1-loaded strips of ureteric muscles. Western blotting and immuno-histochemistry have been used to define the presence of PKCα in ureter muscle. Using Western blotting and immuno-histochemistry we found that PKCα was expressed in both guinea-pig and rat ureteric smooth muscles. In small bundles of ureter activation of PKC by low concentrations of PDBU (100nM) produced a marked increase in the duration of the Ca²⁺ transient (3.2±0.4 times, n=7, recorded using Indo-1), and the amplitude and duration of the phasic contractions (1.9±0.1 times and 3.4±0.3 times, respectively, n=7). These effects of PDBU were blocked by two inhibitors of the PKC, GF109203X (5μM) or RO-32-0432 (5μM) (n=5). Neither of these PKC inhibitors had any significant effects on the Ca²⁺ transients and phasic contractions evoked by EFS in the absence of PDBU (n=7). Inhibition of the SR Ca²⁺-ATPase by cyclopiazonic acid (20µM) (n=4) or inhibition of BK_Ca channels by low concentrations of TEA (1mM) (n=6), did not prevent the stimulant action of PDBU on the Ca²⁺ transients and phasic contractions. These data therefore suggest that the stimulatory action of PDBU on guinea-pig ureteric smooth muscle can not be explained by inhibition of a Ca²⁺ sparks/STOCs (spontaneous transient outward currents via BK channels) coupling mechanism reported for vascular smooth muscle (1). Pretreatment of the ureteric smooth muscle cells with the L-type Ca²⁺ channel agonist BayK 8644 (1μM) strongly inhibited the stimulant action of PDBU on ureteric smooth muscle suggesting that L-type Ca²⁺ channels are likely to be targeted by PKC (n=3). The stimulation of PDBU on the ureteric cells was strongly inhibited by intracellular acidification and potentiated by intracellular alkalinization in the guinea pig ureter (n=5). These data suggest that PKC can play an important role in control of phasic contractions in ureter muscle mediated by possible modulation of the L-type Ca²⁺ channels and that these effects of PKC are critically dependent on the intracellular pH.