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 (I(K(Ca))) and the ratio of fura-2 fluorescence (R(340/380)) were considered to reflect the [Ca2+](s1) and [Ca2+](c), respectively. At a holding potential of 0 mV, extracellular application of 10 mM caffeine, a well-known Ca2+-releasing agent, induced transient increase of I(K(Ca)) and R(340/380) (I(K(Ca))-transient and R(340/380)-transient, respectively). The increase and decay of I(K(Ca))-transient was faster than R(340/380)-transient. By repetitive application of caffeine, when the refilling state of SR was supposed to be lower than the control condition, I(K(Ca))-transient and R(340/380)-transient were suppressed to different levels; e.g. the second application 20 sec after the first could induce smaller I(K(Ca))-transient than R(340/380)-transient. Dissociation of I(K(Ca))-transient and R(340/380)- transient was removed by sufficient (>3 min) washout of caffeine. Recovery from the dissociation was also dependent upon the membrane potential; faster recovery was observed at negative (-40 mV) holding potential than at depolarized (0 mV) condition. Dissociation of I(K(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+.
|Number of pages||14|
|Journal||Journal of Smooth Muscle Research|
|State||Published - 1 Dec 1998|
- Ca concentration
- Ca-activated K current
- Smooth muscle