ARTICLES

Cell Research (2004); 14(5): 379-388

Spontaneous Ca²⁺ oscillations in subcellular compartments of vascular smooth muscle cells rely on different Ca²⁺ pools

Olesya D. Fedoryak¹, Yvonne Searls^1,2, Irina V. Smirnova¹, Douglas M. Burns³, Lisa Stehno-Bittel^1,2*

¹Departments of Physical Therapy and Rehabilitation Sciences, University of Kansas Medical Center, Kansas City, KS 66160, USA.
²Department of Anatomy and Cell Biology, University of Kansas Medical Center, Kansas City, KS 66160,USA.
³VA Medical Center, Kansas City, MO 64128,USA

Received, Apr 26, 2004    Revised, Jun 28, 2004    Accepted, Jul 3, 2004

Correspondence: Lisa Stehno-Bittel (01) 913-588-4570 (phone) (01) 913-588-4568 (fax) lbittel@kumc.edu

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Spontaneous Ca²⁺ oscillations in vascular smooth muscle cells have been modeled using a single Ca²⁺ pool. This report describes spontaneous Ca²⁺ oscillations dependent on two separate Ca²⁺ sources for the nuclear versus cytoplasmic compartments. Changes in free intracellular Ca²⁺ were monitored with ratiometric Ca²⁺- fluorophores using confocal microscopy. On average, spontaneous oscillations developed in 79% of rat aortic smooth muscle cells that were synchronous between the cytoplasm and nucleus. Reduction of extracellular Ca²⁺ (< 1 mM) decreased the frequency and amplitude of the cytoplasmic oscillations with 48% of the oscillations asynchronous between the nuclear and cytoplasmic compartments. Similar results were obtained with the Ca²⁺ channel blockers, nimodipine and diltiazem. Arg-vasopressin (AVP) induced a rapid release of intracellular Ca²⁺ stores that was greater in the nuclear compartment (4.20 ± 0.23 ratio units, n = 56) than cytoplasm (2.54 ± 0.28) in cells that had spontaneously developed prior oscillations. Conversely, cells in the same conditions lacking oscillations had a greater AVP-induced Ca²⁺ transient in the cytoplasm (4.99 ± 0.66, n = 17) than in the nucleus (2.67 ± 0.29). Pre-treatment with Ca²⁺ channel blockers depressed the AVP responses in both compartments with the cytoplasmic Ca²⁺ most diminished. Depletion of internal Ca²⁺ stores prior to AVP exposure blunted the nuclear response, mimicking the response of cells that lacked prior oscillations. Spontaneous oscillating cells had a greater sarcoplasmic reticulum network than cells that did not oscillate. We propose that spontaneous nuclear oscillations rely on perinuclear sarcoplasmic reticulum stores, while the cytoplasmic oscillations rely on Ca²⁺ influx.

Keywords: oscillations, vascular smooth muscle cells, nuclear Ca²⁺, vasopressin, thapsigargin, diltiazem, nimodipine.