Sarcoplasmic reticulum buffering of myoplasmic calcium in bovine coronary artery smooth muscle

Michael Sturek, K. Kunda, Q. Hu

Research output: Contribution to journalArticle

39 Citations (Scopus)

Abstract

1. We tested the hypothesis that the sarcoplasmic reticulum (SR) buffers (attenuates) the increase in averaged myoplasmic free [Ca2+] (Ca(im)) resulting from Ca2+ influx. 2. Fura-2 measurements of Ca(im) were obtained in single smooth muscle cells freshly dispersed from bovine coronary artery. 3. Caffeine (5 x 10-3 M) elicited a transient increase in Ca(im) and depleted the SR Ca2+ store. In the continued presence of caffeine or 10-5 M-ryanodine SR buffering of Ca(im) was inhibited. Subsequent exposure to high extracellular [K+] (> 30 mM, equimolar Na+ removal) elicited a 2-fold more rapid and 2-fold greater peak increase in Ca(im) than high K+ elicited when SR buffering of Ca(im) was normal. The augmented increase in Ca(im) was inhibited 35% by 10-5 M-diltiazem, 65% by 2 x 10-4 M-LaCl3, and 87% in Ca2+-free external solution. 4. When Ca(im) buffering capacity was increased by partially depleting the SR with a transient (1 min) exposure to caffeine, subsequent exposure to 80 mM-K+ solution increased Ca(im) almost 2-fold more slowly than 80 mM-K+ before depletion of Ca2+ from the SR. However, the influxing Ca2+ was sequestered by the SR and refilled it, as evident by the subsequent caffeine-induced Ca(im) transient being identical to the first. Increasing extracellular [K+] (thus, increasing depolarization and Na+ removal) caused proportional increases in Ca(im) and the subsequent caffeine-induced Ca(im) transients were proportionally larger, indicating a graded filling of the SR by Ca2+ influx. 5. Diltiazem (10-5 M) inhibited the refilling of the SR achieved by 80 mM-K+, by 26%. Refilling was inhibited 76% by 80 mM-K+, Ca2+-free solution, indicating the fraction of refilling dependent on influx of Ca2+ through voltage-gated Ca2+ channels, leak channels, and other influx pathways. Mild depolarization with 35 mM-K+ (no Na+ removal) often caused no increase in Ca(im), but influx through voltage-gated Ca2+ channels occurred because the SR Ca2+ store was refilled. Also, 10-5 M-diltiazem or 10-6 M-TA3090 inhibited the refilling to levels attributable only to leak influx of Ca2+ 6. All data support our hypothesis that the SR significantly attenuates the amount of Ca2+ influx that accumulates to increase Ca(im). The main implications are: (a) vasoactive agents that cause external Ca2+-dependent increases in Ca(im) may inhibit SR buffering of Ca(im), rather than increasing Ca2+ influx across the sarcolemma; (b) classical voltage-gated Ca2+ channel antagonists may also act by inhibiting both localized increases in Ca2+ in a restricted subsarcolemmal compartment and refilling of the SR, while not affecting Ca(im).

Original languageEnglish (US)
Pages (from-to)25-48
Number of pages24
JournalJournal of Physiology
Volume451
StatePublished - 1992
Externally publishedYes

Fingerprint

Sarcoplasmic Reticulum
Smooth Muscle
Coronary Vessels
Calcium
Caffeine
Diltiazem
Ryanodine
Sarcolemma
Fura-2
Smooth Muscle Myocytes
Buffers

ASJC Scopus subject areas

  • Physiology

Cite this

Sarcoplasmic reticulum buffering of myoplasmic calcium in bovine coronary artery smooth muscle. / Sturek, Michael; Kunda, K.; Hu, Q.

In: Journal of Physiology, Vol. 451, 1992, p. 25-48.

Research output: Contribution to journalArticle

@article{f127991ecba14a98a8b0afa37464fe2b,
title = "Sarcoplasmic reticulum buffering of myoplasmic calcium in bovine coronary artery smooth muscle",
abstract = "1. We tested the hypothesis that the sarcoplasmic reticulum (SR) buffers (attenuates) the increase in averaged myoplasmic free [Ca2+] (Ca(im)) resulting from Ca2+ influx. 2. Fura-2 measurements of Ca(im) were obtained in single smooth muscle cells freshly dispersed from bovine coronary artery. 3. Caffeine (5 x 10-3 M) elicited a transient increase in Ca(im) and depleted the SR Ca2+ store. In the continued presence of caffeine or 10-5 M-ryanodine SR buffering of Ca(im) was inhibited. Subsequent exposure to high extracellular [K+] (> 30 mM, equimolar Na+ removal) elicited a 2-fold more rapid and 2-fold greater peak increase in Ca(im) than high K+ elicited when SR buffering of Ca(im) was normal. The augmented increase in Ca(im) was inhibited 35{\%} by 10-5 M-diltiazem, 65{\%} by 2 x 10-4 M-LaCl3, and 87{\%} in Ca2+-free external solution. 4. When Ca(im) buffering capacity was increased by partially depleting the SR with a transient (1 min) exposure to caffeine, subsequent exposure to 80 mM-K+ solution increased Ca(im) almost 2-fold more slowly than 80 mM-K+ before depletion of Ca2+ from the SR. However, the influxing Ca2+ was sequestered by the SR and refilled it, as evident by the subsequent caffeine-induced Ca(im) transient being identical to the first. Increasing extracellular [K+] (thus, increasing depolarization and Na+ removal) caused proportional increases in Ca(im) and the subsequent caffeine-induced Ca(im) transients were proportionally larger, indicating a graded filling of the SR by Ca2+ influx. 5. Diltiazem (10-5 M) inhibited the refilling of the SR achieved by 80 mM-K+, by 26{\%}. Refilling was inhibited 76{\%} by 80 mM-K+, Ca2+-free solution, indicating the fraction of refilling dependent on influx of Ca2+ through voltage-gated Ca2+ channels, leak channels, and other influx pathways. Mild depolarization with 35 mM-K+ (no Na+ removal) often caused no increase in Ca(im), but influx through voltage-gated Ca2+ channels occurred because the SR Ca2+ store was refilled. Also, 10-5 M-diltiazem or 10-6 M-TA3090 inhibited the refilling to levels attributable only to leak influx of Ca2+ 6. All data support our hypothesis that the SR significantly attenuates the amount of Ca2+ influx that accumulates to increase Ca(im). The main implications are: (a) vasoactive agents that cause external Ca2+-dependent increases in Ca(im) may inhibit SR buffering of Ca(im), rather than increasing Ca2+ influx across the sarcolemma; (b) classical voltage-gated Ca2+ channel antagonists may also act by inhibiting both localized increases in Ca2+ in a restricted subsarcolemmal compartment and refilling of the SR, while not affecting Ca(im).",
author = "Michael Sturek and K. Kunda and Q. Hu",
year = "1992",
language = "English (US)",
volume = "451",
pages = "25--48",
journal = "Journal of Physiology",
issn = "0022-3751",
publisher = "Wiley-Blackwell",

}

TY - JOUR

T1 - Sarcoplasmic reticulum buffering of myoplasmic calcium in bovine coronary artery smooth muscle

AU - Sturek, Michael

AU - Kunda, K.

AU - Hu, Q.

PY - 1992

Y1 - 1992

N2 - 1. We tested the hypothesis that the sarcoplasmic reticulum (SR) buffers (attenuates) the increase in averaged myoplasmic free [Ca2+] (Ca(im)) resulting from Ca2+ influx. 2. Fura-2 measurements of Ca(im) were obtained in single smooth muscle cells freshly dispersed from bovine coronary artery. 3. Caffeine (5 x 10-3 M) elicited a transient increase in Ca(im) and depleted the SR Ca2+ store. In the continued presence of caffeine or 10-5 M-ryanodine SR buffering of Ca(im) was inhibited. Subsequent exposure to high extracellular [K+] (> 30 mM, equimolar Na+ removal) elicited a 2-fold more rapid and 2-fold greater peak increase in Ca(im) than high K+ elicited when SR buffering of Ca(im) was normal. The augmented increase in Ca(im) was inhibited 35% by 10-5 M-diltiazem, 65% by 2 x 10-4 M-LaCl3, and 87% in Ca2+-free external solution. 4. When Ca(im) buffering capacity was increased by partially depleting the SR with a transient (1 min) exposure to caffeine, subsequent exposure to 80 mM-K+ solution increased Ca(im) almost 2-fold more slowly than 80 mM-K+ before depletion of Ca2+ from the SR. However, the influxing Ca2+ was sequestered by the SR and refilled it, as evident by the subsequent caffeine-induced Ca(im) transient being identical to the first. Increasing extracellular [K+] (thus, increasing depolarization and Na+ removal) caused proportional increases in Ca(im) and the subsequent caffeine-induced Ca(im) transients were proportionally larger, indicating a graded filling of the SR by Ca2+ influx. 5. Diltiazem (10-5 M) inhibited the refilling of the SR achieved by 80 mM-K+, by 26%. Refilling was inhibited 76% by 80 mM-K+, Ca2+-free solution, indicating the fraction of refilling dependent on influx of Ca2+ through voltage-gated Ca2+ channels, leak channels, and other influx pathways. Mild depolarization with 35 mM-K+ (no Na+ removal) often caused no increase in Ca(im), but influx through voltage-gated Ca2+ channels occurred because the SR Ca2+ store was refilled. Also, 10-5 M-diltiazem or 10-6 M-TA3090 inhibited the refilling to levels attributable only to leak influx of Ca2+ 6. All data support our hypothesis that the SR significantly attenuates the amount of Ca2+ influx that accumulates to increase Ca(im). The main implications are: (a) vasoactive agents that cause external Ca2+-dependent increases in Ca(im) may inhibit SR buffering of Ca(im), rather than increasing Ca2+ influx across the sarcolemma; (b) classical voltage-gated Ca2+ channel antagonists may also act by inhibiting both localized increases in Ca2+ in a restricted subsarcolemmal compartment and refilling of the SR, while not affecting Ca(im).

AB - 1. We tested the hypothesis that the sarcoplasmic reticulum (SR) buffers (attenuates) the increase in averaged myoplasmic free [Ca2+] (Ca(im)) resulting from Ca2+ influx. 2. Fura-2 measurements of Ca(im) were obtained in single smooth muscle cells freshly dispersed from bovine coronary artery. 3. Caffeine (5 x 10-3 M) elicited a transient increase in Ca(im) and depleted the SR Ca2+ store. In the continued presence of caffeine or 10-5 M-ryanodine SR buffering of Ca(im) was inhibited. Subsequent exposure to high extracellular [K+] (> 30 mM, equimolar Na+ removal) elicited a 2-fold more rapid and 2-fold greater peak increase in Ca(im) than high K+ elicited when SR buffering of Ca(im) was normal. The augmented increase in Ca(im) was inhibited 35% by 10-5 M-diltiazem, 65% by 2 x 10-4 M-LaCl3, and 87% in Ca2+-free external solution. 4. When Ca(im) buffering capacity was increased by partially depleting the SR with a transient (1 min) exposure to caffeine, subsequent exposure to 80 mM-K+ solution increased Ca(im) almost 2-fold more slowly than 80 mM-K+ before depletion of Ca2+ from the SR. However, the influxing Ca2+ was sequestered by the SR and refilled it, as evident by the subsequent caffeine-induced Ca(im) transient being identical to the first. Increasing extracellular [K+] (thus, increasing depolarization and Na+ removal) caused proportional increases in Ca(im) and the subsequent caffeine-induced Ca(im) transients were proportionally larger, indicating a graded filling of the SR by Ca2+ influx. 5. Diltiazem (10-5 M) inhibited the refilling of the SR achieved by 80 mM-K+, by 26%. Refilling was inhibited 76% by 80 mM-K+, Ca2+-free solution, indicating the fraction of refilling dependent on influx of Ca2+ through voltage-gated Ca2+ channels, leak channels, and other influx pathways. Mild depolarization with 35 mM-K+ (no Na+ removal) often caused no increase in Ca(im), but influx through voltage-gated Ca2+ channels occurred because the SR Ca2+ store was refilled. Also, 10-5 M-diltiazem or 10-6 M-TA3090 inhibited the refilling to levels attributable only to leak influx of Ca2+ 6. All data support our hypothesis that the SR significantly attenuates the amount of Ca2+ influx that accumulates to increase Ca(im). The main implications are: (a) vasoactive agents that cause external Ca2+-dependent increases in Ca(im) may inhibit SR buffering of Ca(im), rather than increasing Ca2+ influx across the sarcolemma; (b) classical voltage-gated Ca2+ channel antagonists may also act by inhibiting both localized increases in Ca2+ in a restricted subsarcolemmal compartment and refilling of the SR, while not affecting Ca(im).

UR - http://www.scopus.com/inward/record.url?scp=0026625505&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=0026625505&partnerID=8YFLogxK

M3 - Article

VL - 451

SP - 25

EP - 48

JO - Journal of Physiology

JF - Journal of Physiology

SN - 0022-3751

ER -