Relationship between global cytosolic Ca2+ concentration and Ca2+-activated K+ current in rabbit cerebral arterial myocyte.

In smooth muscle cells, the sarcoplasmic reticulum (SR) has been identified as the primary storage site for intracellular Ca2+. The peripheral SR is in close proximity with plasma membrane to make a narrow subsarcolemmal space. In this study, we investigated the regulation of subsarcolemmal [Ca2+] ([Ca2+] s1) and global cytosolic [Ca2+] ([Ca2+] c) of rabbit arterial smooth muscle using whole-cell patch clamp technique and microspectrofluorimetry. The Ca2+-activated K+ current (IK (Ca) ) and the ratio of fura-2 fluorescence (R340/380) were considered to reflect the [Ca2+] s1 and [Ca2+] c, respectively. At a holding potential of 0mV, extracellular application of 10mM caffeine, a well-known Ca2+-releasing agent, induced transient increase of IK (Ca) and R340/380 (IK (Ca) -transient and R340/380-transient, respectively). The increase and decay of IK (Ca) -transient was faster than R340/380-transient. By repetitive application of caffeine, when the refilling state of SR was supposed to be lower than the control condition, IK (Ca) -transient and R340/380-transient were suppressed to different levels; e.g. the second application 20sec after the first could induce smaller Ik (Ca) -transient than R340/380-transient. Dissociation of Ik (Ca) -transient and R340/380-transient was removed by sufficient (>3min) washout of caffeine. Recovery from the dissociation was also dependent upon the membrane potential; faster recovery was observed at negative (-40mV) holding potential than at depolarized (0mV) condition. Dissociation of IK (Ca) from [Ca2+] c was also partially prevented by perfusion with Na+-free (replaced by NMDG+) extracellular solution. These results suggest that, 1) there is prominent spatial inhomogeneity of [Ca2+] in cerebral arterial myocyte, 2) [Ca2+] s1 is preferentially affected by the interference from nearby plasmalemmal Ca2+ regulation mechanism which is partly dependent upon extracellular Na+.