1. The aim of this study was to determine whether the site of action of
relaxin as a relaxant of rat myometrium is at the cell membrane or at an intracellular-site. Therefore, the potency of
relaxin was determined against
spasms reliant predominantly upon either extracellular Ca2+ or intracellular Ca2+. Uterine
spasms dependent upon extracellular Ca2+ were elicited by (i)
oxytocin (0.2 nM) (ii)
Bay K 8644 (1 microM) in 10 mM K(+)-rich PSS and (iii) KCl (80 mM). Uterine
spasm dependent upon intracellular Ca2+ was elicited by
oxytocin (20 nM) in the presence of
nifedipine (500 nM). The effects of
relaxin against these spasmogens were compared with those of
levcromakalim,
nifedipine and
salbutamol. 2.
Relaxin (0.2-6.3 nM),
levcromakalim (25-800 nM),
salbutamol (1-63 nM) and
nifedipine (1-250 nM) caused concentration-dependent inhibition of the
spasm evoked by
oxytocin (0.2 nM) and
relaxin was the most potent relaxant. 3.
Relaxin and
nifedipine were slightly less potent against the
spasm induced by
Bay K 8644 (1 microM) than against
spasm induced by
oxytocin (0.2 nM) (15 fold and 13 fold respectively).
Levcromakalim and
salbutamol were equipotent against the
spasm evoked by
Bay K 8644 (1 microM) and that evoked by
oxytocin (0.2 nM). 4.
Relaxin induced only 47 +/- 7% inhibition of the KCl (80 mM)-evoked
spasm at a concentration of 0.8 microM.
Levcromakalim was much less potent (427 fold) against the
spasm evoked by KCl (80 mM) than against the
spasm evoked by
oxytocin (0.2 nM). The potency of
salbutamol against the
spasm evoked by KCl (80 mM) was modestly reduced (14 fold) compared to that against the
spasm evoked by
oxytocin (0.2 nM). The potency of
nifedipine against the KCl (80 mM)-evoked
spasm was not different from that against the
oxytocin (0.2 nM)-evoked
spasm. 5. The potencies of
relaxin and
levcromakalim against the
spasm evoked by
oxytocin (20 nM) +
nifedipine (500 nM) were greatly reduced (74 fold and 234 fold respectively) compared to their potencies against the
spasm evoked by
oxytocin (0.2 nM). The potency of
salbutamol against these two spasmogens was not different. 6.
Relaxin was much less potent against the
spasm dependent upon intracellular Ca2+ (that induced by
oxytocin (20 nM) +
nifedipine (500 nM)) than against the
spasms dependent upon extracellular Ca2+, those induced by
oxytocin (0.2 nM) and
Bay K 8644 (1 microM). In this regard,
relaxin resembled
levcromakalim and
nifedipine rather than
salbutamol. Therefore, the major site of action of
relaxin appears to be located at the plasma membrane rather than at an intracellular level. The observation that
relaxin was less effective against the KCl (80 mM)-induced
spasm than against the
oxytocin (0.2 nM)-evoked
spasm may indicate that
relaxin has a minor action involving K(+)-channel opening. 7. High concentrations of
relaxin (up to 1 microM) induced significant inhibition of the
spasm dependent upon intracellular Ca2+. Thus at high concentrations
relaxin also appears to have an additional intracellular action.