<PAGE>


                                                                     EXHIBIT 4.3


                                 AMENDMENT NO. 1
               TO AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT


         THIS AMENDMENT NO. 1 TO AMENDED AND RESTATED INVESTORS' RIGHTS
AGREEMENT ("Amendment") is made and entered into as of August 11, 1999, by and
among SYMBION, INC., a Tennessee corporation formerly named "UniPhy Healthcare,
Inc." (the "Company"), and those Persons identified on Exhibit A attached hereto
(collectively, the "Investors" and each individually an "Investor").

                                R E C I T A L S:

         WHEREAS, the Company and the Investors are parties to that certain
Amended and Restated Investors' Rights Agreement, dated as of June 25, 1999 (the
"Investors' Rights Agreement");

         WHEREAS, the Company and certain of the Investors are parties to that
certain Securities Purchase Agreement, dated as of July 6, 1999 (the "Purchase
Agreement");

         WHEREAS, Section 1(B) of the Purchase Agreement grants an option (the
"Option") of limited duration to Richland Ventures III, L.P. ("RVIII") to
purchase up to 3,191,489 shares (the "Option Shares") of the common stock, no
par value (the "Common Stock"), of the Company, at a purchase price of $1.88 per
share;

         WHEREAS, in the event that RVIII purchases the Option Shares, the
Company and the Investors desire that RVIII obtain the rights and obligations of
an "Investor," an "ARC Key Shareholder" and an "Anti-Dilution Investor" under
the Investors' Rights Agreement;

         WHEREAS, the Company may, pursuant to Section 1(C) of the Purchase
Agreement, issue and sell up to 5,319,149 shares (the "Reserve Shares") of
Common Stock (including the Option Shares) to any person or entity (each, a
"Reserve Share Purchaser");

         WHEREAS, in the event that a Reserve Share Purchaser purchases Reserve
Shares, the Company and the Investors may desire that such Reserve Share
Purchaser have the rights and obligations of an "Investor," a "UniPhy Preferred
Shareholder," a "UniPhy Founder" or an "ARC Key Shareholder," and an
"Anti-Dilution Investor" under the Investors' Rights Agreement;

         WHEREAS, the Company may, from time to time after the date hereof,
issue and sell additional shares of Common Stock ("Additional Shares") to
various persons or entities (each, a "Subsequent Purchaser");

         WHEREAS, in the event that a Subsequent Purchaser purchases Additional
Shares, the Company and the Investors may desire that such Subsequent Purchaser
have the rights and obligations of an "Investor" and a "UniPhy Preferred
Shareholder," a "UniPhy Founder" or an "ARC Key Shareholder" under the
Investors' Rights Agreement;

         WHEREAS, the Company and the Investors desire to amend the Investors'
Rights Agreement as set forth in this Amendment, pursuant to Section 6.5 of the
Investors' Rights Agreement; and

         WHEREAS, capitalized terms used herein without definition shall have
the same


<PAGE>

meanings ascribed to such terms in the Investors' Rights Agreement.

         NOW, THEREFORE, in consideration of the foregoing and the respective
covenants and agreements contained herein, the sufficiency of which is hereby
acknowledged, the parties hereby agree and consent to the following:

                               A G R E E M E N T:

1.       RVIII Amendment. The Investors' Rights Agreement is hereby amended by
adding and appending the following as Section 7 of the Investors' Rights
Agreement:

         7.   Addition of RVIII as a Party. Notwithstanding the provisions of
         Section 6.5 or any other Section hereof, upon the occurrence of: (i)
         the exercise by Richland Ventures III, L.P. ("RVIII") of the option
         (the "Option") to purchase up to 3,191,489 shares (the "Option Shares")
         of Common Stock at a purchase price of $1.88 per share which was
         granted to RVIII in Section 1(B) of that certain Securities Purchase
         Agreement, dated as of July 6, 1999 (the "Purchase Agreement"), among
         the Company and certain purchasers identified in such agreement, and
         the purchase of Option Shares by RVIII, and (ii) the execution and
         delivery to the Company by RVIII of a counterpart of this Amended and
         Restated Agreement (as then amended), adopting and accepting the terms
         hereof, and (iii) acceptance and countersignature of such counterpart
         by the Company:

              (a) RVIII shall be a party to this Amended and Restated Agreement;
and

              (b) Exhibit A attached to this Amended and Restated Agreement
         shall be amended to add RVIII as a person identified thereon, and RVIII
         shall be, and shall be included within the definition of, an "Investor"
         for purposes of this Amended and Restated Agreement; and

              (c) Exhibit A attached to this Amended and Restated Agreement
         shall be amended to identify RVIII as an "Anti-Dilution Investor," and
         RVIII shall be, and shall be included within the definition of, an
         "Anti-Dilution Investor" for purposes of this Amended and Restated
         Agreement; and

              (d) RVIII shall be, and shall be included within the definition
         of, an "ARC Key Shareholder," for purposes of this Amended and Restated
         Agreement; and

              (e) RVIII shall have, and be entitled and subject to, all of the
         rights, benefits, duties and obligations of an "Investor," an "ARC Key
         Shareholder" and an "Anti-Dilution Investor" under this Amended and
         Restated Agreement, subject to the terms hereof.

2.       Reserve Share Purchasers Amendment. The Investors' Rights Agreement is
hereby further amended by adding and appending the following as Section 8, and
Sections 8.1 and 8.2 thereunder, of the Investors' Rights Agreement:

         8.   Addition of Reserve Share Purchasers as Parties.



                                       2
<PAGE>


              8.1 Reserve Share Purchasers. The parties hereto acknowledge and
         agree that, pursuant to Section 1(C) of the Purchase Agreement, the
         Company may issue, offer and sell up to 5,319,149 shares (the "Reserve
         Shares") of Common Stock (including the 3,191,489 Option Shares) to any
         person or entity (each, a "Reserve Share Purchaser"). Notwithstanding
         the provisions of Section 6.5 or any other Section hereof, upon the
         occurrence of: (i) the purchase from the Company by a Reserve Share
         Purchaser of Reserve Shares (but not including the purchase of Option
         Shares by RVIII, which is addressed in and governed by Section 7
         above), and (ii) the execution and delivery to the Company by such
         Reserve Share Purchaser of a counterpart of this Amended and Restated
         Agreement (as then amended), adopting and accepting the terms hereof,
         and (iii) acceptance and countersignature of such counterpart by the
         Company:

                  (a) such Reserve Share Purchaser shall be a party to this
         Amended and Restated Agreement; and

                  (b) Exhibit A attached to this Amended and Restated Agreement
         shall be amended to add such Reserve Share Purchaser as a person
         identified thereon, and such Reserve Share Purchaser shall be, and
         shall be included within the definition of, an "Investor" for purposes
         of this Amended and Restated Agreement; and

                  (c) subject to Section 8.2 below, Exhibit A attached to this
         Amended and Restated Agreement shall be amended to identify such
         Reserve Share Purchaser as an "Anti-Dilution Investor," and such
         Reserve Share Purchaser shall be, and shall be included within the
         definition of, an "Anti-Dilution Investor" for purposes of this Amended
         and Restated Agreement; and

                  (d) subject to Section 8.2 below, such Reserve Share Purchaser
         shall be, and shall be included within the definition of, either of a
         "UniPhy Preferred Shareholder," a "UniPhy Founder" or an "ARC Key
         Shareholder" (as designated by the Company pursuant to Section 8.2
         below), for purposes of this Amended and Restated Agreement; and

                  (e)   subject to Section 8.2 below, such Reserve Share
         Purchaser shall have, and shall be entitled and subject to, all of the
         rights, benefits, duties and obligations of an "Investor," a "UniPhy
         Preferred Shareholder," a "UniPhy Founder" or an "ARC Key Shareholder"
         (as designated by the Company pursuant to Section 8.2 below), and an
         "Anti-Dilution Investor" under this Amended and Restated Agreement,
         subject to the terms hereof.

              8.2 Designation as Anti-Dilution Investor and UniPhy Preferred
         Shareholder, UniPhy Founder or ARC Key Shareholder. The Company shall
         have the right, power and authority, to determine, in its discretion,
         in negotiation or agreement with one or more Reserve Share Purchasers
         or otherwise, whether or not a particular Reserve Share Purchaser shall
         (in addition to becoming a party hereto and an "Investor" hereunder) be
         identified and/or designated as an "Anti-Dilution Investor" and/or a
         "UniPhy Preferred Shareholder," "UniPhy Founder" or "ARC Key
         Shareholder" for purposes of this Amended and Restated Agreement, and



                                       3
<PAGE>

         thereby have, and be entitled and subject to, all of the rights,
         benefits, duties and obligations of such designation or identification
         under this Amended and Restated Agreement, pursuant to Sub-sections
         8.1(c), (d) and (e) above.

3.       Subsequent Purchasers Amendment. The Investors' Rights Agreement is
hereby further amended by adding and appending the following as Section 9 of the
Investors' Rights Agreement:

         9. Addition of Subsequent Purchasers as Parties. The parties hereto
         acknowledge and agree that the Company may, from time to time, offer
         and sell shares of Common Stock (such shares, "Additional Shares") in
         addition to Option Shares and Reserve Shares to one or more
         individuals, persons or entities (each, a "Subsequent Purchaser").
         Notwithstanding the provisions of Section 6.5 or any other Section
         hereof, upon the occurrence of: (i) the purchase from the Company by a
         Subsequent Purchaser of Additional Shares, and (ii) the execution and
         delivery to the Company by such Subsequent Purchaser of a counterpart
         of this Amended and Restated Agreement (as then amended), adopting and
         accepting the terms hereof, and (iii) acceptance and countersignature
         of such counterpart by the Company:

              (a) such Subsequent Purchaser shall be a party to this Amended and
         Restated Agreement; and

              (b) Exhibit A attached to this Amended and Restated Agreement
         shall be amended to add such Subsequent Purchaser as a person
         identified thereon, and such Subsequent Purchaser shall be, and shall
         be included within the definition of, an "Investor" for purposes of
         this Amended and Restated Agreement; and

              (c) such Subsequent Purchaser shall have, and shall be entitled
         and subject to, all of the rights, benefits, duties and obligations of
         an "Investor" under this Amended and Restated Agreement, subject to the
         terms hereof.

4.       Deletion of Current Provision Regarding Additional Investors. The
Investors' Rights Agreement is hereby further amended by deleting Sub-section
6.5(c) of the Investors' Rights Agreement in its entirety, which subsection
shall thereupon be void and without effect.

5.       Requisite Approval. Upon execution and delivery of this Amendment by
the Company and Investors representing the requisite percentage of Registrable
Shares set forth in Section 6.5 of the Investors' Rights Agreement, this
Amendment shall constitute and be deemed to be an amendment to and part of the
Investors' Rights Agreement.

6.       No Other Changes. Except as expressly set forth or contemplated in this
Amendment, the terms and conditions of the Investors' Rights Agreement shall
remain in place and shall not be altered, amended or changed in any manner
whatsoever, except by any further amendment to the Investors' Rights Agreement
made in accordance with the terms of the Investors' Rights Agreement, as hereby
amended.

7.       Counterparts. This Amendment may be executed in any number of
counterparts and by the different parties hereto on separate counterparts, each
of which when executed and delivered shall be deemed an original effective for
binding the parties hereto, but all of which shall together constitute one and
the same instrument.


                                       4
<PAGE>


         IN WITNESS WHEREOF, the parties hereto have executed this Amendment to
Amended and Restated Investors' Rights Agreement, and hereby consent to the
amendment and modification of the Investors' Rights Agreement provided herein,
as of the date first above written.


THE COMPANY:

SYMBION, INC.


By: /s/ Richard E. Francis, Jr.
    ----------------------------------------
Name: Richard E. Francis, Jr.
Title: President and Chief Executive Officer



INVESTORS:


UniPhy PREFERRED SHAREHOLDERS:

ABS Capital Partners, L.P.

/s/ ABS Partners, L.P.
--------------------------------------------
By: /s/ Frederick L. Bryant
    ----------------------------------------
Name: Frederick L. Bryant
     ---------------------------------------
Title: General Partner
       -------------------------------------


/s/ Clifford G. Adlerz
--------------------------------------------
Clifford G. Adlerz


/s/ Randy W. Cook
--------------------------------------------
Randy W. Cook


--------------------------------------------
Paul Davis


/s/ Richard E. Francis, Jr.
--------------------------------------------
Richard E. Francis, Jr.


/s/ R. Dale Kennedy
--------------------------------------------
R. Dale Kennedy



                                       5
<PAGE>


--------------------------------------------
Tod Lambert

MARTIN INVESTMENT PARTNERSHIP II

/s/ Charles N. Martin, Jr.
--------------------------------------------
Name: Charles N. Martin, Jr.
      --------------------------------------
Title:
       -------------------------------------


--------------------------------------------
Kenneth C. Mitchell


PACIFIC VENTURE GROUP, L.P.
By:      PVG Equity Partner, L.L.C.,
         its General Partner

/s/ Eve M. Kurtin
--------------------------------------------
By: Eve M. Kurtin
Title: Member


PVG ASSOCIATES, L.P.
By:      PVG Equity Partner, L.L.C.,
         its General Partner

/s/ Eve M. Kurtin
--------------------------------------------
By: Eve M. Kurtin
Title: Member


/s/ Patrick R. Rooney
--------------------------------------------
Patrick R. Rooney


/s/ Teresa F. Sparks
--------------------------------------------
Teresa F. Sparks


/s/ Gregg Stanley
--------------------------------------------
Gregg Stanley


/s/ Anthony Taparo
--------------------------------------------
Anthony Taparo


ARC KEY SHAREHOLDERS:



                                       6
<PAGE>


ARTHUR S. DEMOSS FOUNDATION


--------------------------------------------
By:
    ----------------------------------------
Name:
     ---------------------------------------
Title:
       -------------------------------------


/s/ Jerry M. Eyler
--------------------------------------------
Jerry M. Eyler


FIRST AVENUE PARTNERS, L.P.

/s/ David M. Wilds
--------------------------------------------
Name: David M. Wilds
     ---------------------------------------
Title:
       -------------------------------------



--------------------------------------------
William H. Freeman



--------------------------------------------
J. Michael Gould


HEALTH TECH AFFILIATES, INC.


--------------------------------------------
By:
    ----------------------------------------
Name:
     ---------------------------------------
Title:
       -------------------------------------


HLM/CB FUND, L.P.


--------------------------------------------
By:
    ----------------------------------------
Name:
     ---------------------------------------
Title:
       -------------------------------------


                                       7

<PAGE>


HLM PARTNERS VII, L.P.


--------------------------------------------
By:
    ----------------------------------------
Name:
     ---------------------------------------
Title:
       -------------------------------------


--------------------------------------------
W. Lawrence Hough


PIPER JAFFRAY, INC.

By:
    ----------------------------------------
Name:
     ---------------------------------------
Title:
       -------------------------------------



--------------------------------------------
B. Max Lauderdale


/s/ Charles N. Martin, Jr.
--------------------------------------------
Charles N. Martin, Jr.


/s/ Charles T. Neal
--------------------------------------------
Charles T. Neal



--------------------------------------------
Richard Panek



--------------------------------------------
Thomas Patten



--------------------------------------------
Keith B. Pitts



--------------------------------------------
Gary J. Prosterman



RICHLAND VENTURES II, L.P.




                                       8
<PAGE>

/s/ Jack Tyrrell
--------------------------------------------
Name: Jack Tyrrell
     ---------------------------------------
Title: Partner
       -------------------------------------



--------------------------------------------
Ann R. Schaaf


SOUTH ATLANTIC PRIVATE EQUITY FUND IV (QP), LIMITED PARTNERSHIP
         By:     South Atlantic Private Equity Partners IV, Limited Partnership,
                 its General Partner

                 /s/ Donald W. Burton
                 --------------------------------------------
                 Name: Donald W. Burton
                       --------------------------------------
                 Title: Chairman
                        -------------------------------------


SOUTH ATLANTIC PRIVATE EQUITY FUND IV, L.P.
         By:     South Atlantic Private Equity Partners IV, Limited Partnership,
                 its General Partner

                 /s/ Donald W. Burton
                 --------------------------------------------
                 Name: Donald W. Burton
                       --------------------------------------
                 Title: Chairman
                        -------------------------------------



--------------------------------------------
Cal Turner, Jr.


/s/ William V.B. Webb
--------------------------------------------
William V.B. Webb


WW TWO, L.P.


By:
    ----------------------------------------
Name:
     ---------------------------------------
Title:
       -------------------------------------


--------------------------------------------
James A. Webb, III


                                       9
<PAGE>


UniPhy FOUNDERS:




--------------------------------------------
Peter Davidson


/s/ Shannon W. Fiser
--------------------------------------------
Shannon W. Fiser


THE FRANCIS IRREVOCABLE TRUST, ANN FRANCIS, TRUSTEE

/s/ Richard E. Francis, Jr.
--------------------------------------------
Name: Richard E. Francis, Jr.
     ---------------------------------------
Title:
       -------------------------------------


/s/ Victor Giovanetti
--------------------------------------------
Victor Giovanetti


/s/ George M. Goodwin
--------------------------------------------
George M. Goodwin



--------------------------------------------
Marlene D. Kelley


/s/ H. Edward Kleine III
--------------------------------------------
H. Edward Kleine III



--------------------------------------------
Darrell Naish



--------------------------------------------
Richard Panek


/s/ Vickie R. Warren
--------------------------------------------
Vickie R. Warren


                                    EXHIBIT A

                              SCHEDULE OF INVESTORS



                                       10
<PAGE>

UniPhy PREFERRED SHAREHOLDERS

ABS Capital Partners, L.P.*
Clifford G. Adlerz*
Randy W. Cook*
Paul Davis*
Richard E. Francis, Jr.*
R. Dale Kennedy*
Tod Lambert
Martin Investment Partnership II*
Kenneth C. Mitchell*
Pacific Venture Group, L.P.*
PVG Associates, L.P.*
Patrick R. Rooney*
Teresa F. Sparks*
Gregg Stanley*
Anthony Taparo*

ARC KEY SHAREHOLDERS

Arthur S. Demoss Foundation*
Jerry M. Eyler*
First Avenue Partners, L.P.*
William H. Freeman
J. Michael Gould
Health Tech Affiliates, Inc.
HLM/CB Fund, L.P.*
HLM Partners VII, L.P.*
W. Lawrence Hough
Piper Jaffray, Inc.*
B. Max Lauderdale
Charles N. Martin, Jr.*
Charles T. Neal*
Thomas Patten
Keith B. Pitts
Gary J. Prosterman
Richland Ventures II, L.P.*
Ann R. Schaaf
South Atlantic Private Equity Fund IV (QP)*
South Atlantic Private Equity Fund IV, L.P.*
Cal Turner, Jr.*
William V.B. Webb*
WW Two, L.P.*
James A. Webb, III

*   ANTI-DILUTION INVESTOR

UniPhy FOUNDERS



                                       11
<PAGE>


Peter Davidson
Shannon W. Fiser
The Francis Irrevocable Trust, Ann Francis, Trustee*
Victor Giovanetti
George M. Goodwin
Marlene D. Kelley
H. Edward Kleine III
Darrell Naish
Richard Panek
Vickie R. Warren






*  ANTI-DILUTION INVESTOR



                                       12