Pharmacomechanical coupling in rat vas deferens: Effects of agents that modulate intracellular release of calcium and protein kinase C activation

Abstract

The effects of agents that modulate intracellular release of calcium and protein kinase C (PKC) activation on noradrenaline (NA)-induced contractions of epididymal vas deferens in calcium-free/EGTA (1 mM) medium were investigated. NA (100 μM) or methoxamine (100 μM) evoked repeatable contractions. Clonidine (100–300 μM) was ineffective. The contractions to NA were reduced by procaine (1–10 mM) but not by thapsigargin (0.1–30 μM), ryanodine (1–30 μM) or TMB-8 (1–30 μM). Contractions to cumulative additions of NA (1–100 μM) were enhanced in the presence of cyclopiazonic acid (10 & 30 μM) but not ryanodine (10 & 30 μM). Sequential contractions to NA were not blocked by PKC inhibitors, calphostin C (1 μM) or Ro 31-8220 (1–30 μM) but were reduced by H-7 (1–30 μM), a broad spectrum protein kinase inhibitor. Although RT-PCR experiments detected mRNA for some Ca 2+-dependent/DAG-activated and Ca 2+-independent/DAG-activated PKC isoforms in epididymal vas deferens, the PKC activators, phorbol 12, 13-dibutyrate (100 μM) or phorbol 12-myristate 13-acetate (100 μM) failed to activate the tissues in calcium-free medium but enhanced subsequent contractions to NA. These results indicate a limited role for intracellular calcium stores and phorbol ester/DAG-sensitive PKC isoforms in NA-induced contraction of epididymal rat vas deferens in calcium-free medium. The results suggest that pharmacomechanical coupling triggered by NA may involve the sensitization of contractile myofilaments to Ca 2+ or a Ca 2+-independent mechanism. The possible involvement of Ca 2+-independent/DAG-insensitive PKC isoforms and agonist-dependent but PKC-independent sensitization pathway is discussed.